What is KVM? That’s a question many IT professionals and businesses are asking as they look for reliable virtualization solutions. In today’s fast-paced environment, where scalable and flexible infrastructures are more critical than ever, server virtualization has become a key strategy. Understanding technologies like KVM can help you make smarter decisions about how to optimize hardware resources and improve system performance.
This article from Buyserver Magazine gives you a clear, practical overview of what KVM is and how it works.
Definition of KVM Hypervisor
KVM (Kernel-based Virtual Machine) is an open-source virtualization technology that turns the Linux kernel into a type 1 hypervisor. In other words, KVM allows Linux to run virtual machines directly on the physical server—without needing a separate underlying operating system. Since 2007, KVM has been built into the mainline Linux kernel, meaning if you’re running Linux version 2.6.20 or newer, KVM is already included by default.
With KVM, you can divide a physical server into multiple isolated virtual servers. Each virtual machine functions like an independent computer, with its own CPU, RAM, storage, and network interface. This level of isolation helps ensure that one VM doesn’t affect the performance or stability of others.
It’s also worth noting that KVM requires hardware virtualization support—such as Intel VT or AMD-V—to run properly. With this support enabled, KVM can deliver performance that’s very close to running directly on physical hardware.
What is a Hypervisor?
To better understand what KVM is, it’s important to first understand the concept of a hypervisor. Simply put, a hypervisor is the software that enables virtualization. It allows a single physical server to be divided into multiple virtual machines (VMs), each sharing the same hardware resources.
The hypervisor acts as a layer between the physical hardware and the guest operating systems. It’s responsible for managing and allocating resources such as CPU, memory, and storage to each virtual machine. Because of this role, a hypervisor is also known as a Virtual Machine Monitor (VMM).
Types of Hypervisors
There are two main types of hypervisors, each with its own structure and use case:
Type 1 Hypervisor (Bare Metal)
This type runs directly on the physical hardware, without any operating system in between. Because it has direct access to the hardware, it offers excellent performance and strong security. Type 1 hypervisors are typically used in enterprise data centers and cloud environments.
Some well-known examples include KVM, VMware ESXi, and Microsoft Hyper-V.
Type 2 Hypervisor (Hosted)
Type 2 hypervisors are installed as software on top of an existing operating system—such as Windows or Linux. In this case, the OS is installed first, and the hypervisor runs as an application within it.
Because virtual machines must go through the host OS to access hardware, there’s more overhead compared to type 1. These hypervisors are better suited for personal use, development, and testing.
Common examples include VMware Workstation and Oracle VirtualBox.
Virtualization
Virtualization is the process of abstracting physical hardware in a way that allows multiple operating systems to use and share system resources simultaneously and efficiently. Instead of each operating system needing its own dedicated physical machine, virtualization allows you to divide one server’s resources into multiple logical or virtual instances.
For example, a server with 16 CPU cores can be configured to run several virtual machines, each assigned 2 cores. That means you can run multiple operating systems side by side on the same physical hardware, without interference.
By installing a hypervisor, you can create and manage multiple virtual servers that logically segment CPU, memory, storage, and other resources. This approach not only helps reduce hardware costs and save time, but also improves manageability, strengthens data security, and boosts the overall quality of software and service delivery.
Why Use KVM?
You might be wondering: why choose KVM over other virtualization options? There are several solid reasons. The most important is that KVM is open-source and highly flexible. It’s completely free to use and can be modified to suit specific needs. This not only reduces R&D costs but also helps improve system performance and overall efficiency.
Another key advantage is that KVM turns the Linux kernel into a Type 1 hypervisor. This allows developers and system administrators to scale infrastructure and run multiple operating systems—without the need for additional physical hardware. In cloud environments, KVM makes it easy to deploy a large number of virtual machines with speed and stability.
KVM is a full-featured virtualization solution for Linux distributions running on x86 hardware. It supports a wide range of features and works well in both small-scale setups and large enterprise environments.
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Advantages of KVM
As an open-source hypervisor that’s built into the Linux kernel, KVM offers several major benefits:
Free and Open-Source
KVM doesn’t require any licensing fees—it’s completely free. This helps reduce operational costs and gives users the freedom to customize the hypervisor based on their needs.
High Performance
Since KVM runs directly on the Linux kernel and makes use of hardware virtualization extensions, it delivers performance that’s very close to native hardware, with low latency and high efficiency.
Advanced Security
KVM takes advantage of Linux’s built-in security features like SELinux and sVirt, which help enforce strong isolation between virtual machines and prevent unauthorized memory access.
Stability and Reliability
KVM has been used in production environments for well over a decade. It’s backed by a strong open-source community and supported by major organizations such as Red Hat, making it a stable and trustworthy option.
Flexible Resource Management
You can dynamically allocate resources like CPU, memory, and storage to your virtual machines based on real-time needs, which helps optimize system utilization.
Live Migration
KVM supports Live Migration, allowing you to move running virtual machines between physical hosts without any downtime—a crucial feature for high-availability environments.
Wide Hardware Support
Thanks to the close collaboration between hardware vendors and the Linux community, KVM supports a wide range of hardware platforms and stays up-to-date with the latest CPU and chipset features.
Disadvantages of KVM
Like any technology, KVM has its limitations. Here are a few to consider:
Linux Dependency
Because KVM is a Linux kernel module, it only runs on Linux-based systems. This can be a drawback for organizations that rely heavily on Windows-based infrastructure.
Complex Installation and Configuration
For users unfamiliar with Linux or command-line tools, setting up KVM may feel more complicated compared to other hypervisors that offer graphical installers or simpler setup processes.
Hardware Requirements
Running a large number of high-performance virtual machines requires a powerful host system, especially in terms of CPU and RAM. Without sufficient hardware, performance may suffer.
KVM vs. Other Hypervisors
KVM isn’t the only player in the server virtualization space. There are several other well-known hypervisors available, each with its own approach and strengths. To help you understand how KVM compares to its competitors, here’s a quick side-by-side breakdown:
| Feature | KVM | VMware ESXi | Xen | OpenVZ |
| Type of Hypervisor | Type 1 (Bare-Metal) | Type 1 (Bare-Metal) | Type 1 (Bare-Metal) | Container-based |
| License | Free and Open-Source | Commercial (Closed-Source, Paid) | Open-Source (Free & Paid Options) | Free and Open-Source |
| Linux Integration | Native (Built into Linux Kernel) | Independent of Linux Kernel | Independent hypervisor, often on Linux | Based on Linux Kernel (Linux-only) |
| Performance & Isolation | Very high performance, full isolation | Very high performance, full isolation | High performance, full isolation | High performance, but weaker isolation |
| Nested Virtualization | Supported | Supported (in some versions) | Supported (in some versions) | Not supported |
| Primary Use Case | Linux VMs, Cloud, Development | Enterprise Data Centers | Cloud, Virtual Servers | Web hosting, lightweight containers |
| Setup Complexity | Medium (Requires Linux knowledge) | Medium to High (GUI-based tools) | Medium | Low (Simple and fast to deploy) |
Managing the KVM Hypervisor
In the previous section, we explored what KVM is and how it compares to other hypervisors. Now, let’s look at how to manage KVM effectively. Managing a KVM hypervisor involves tasks such as creating, configuring, launching, and monitoring virtual machines. Several tools can help with this:
Libvirt
Libvirt is an open-source library and management layer that provides an interface between KVM and various tools. It offers a stable API for VM management and supports automation of many virtualization tasks.
Virt-Manager
Virt-Manager (Virtual Machine Manager) is a powerful graphical user interface (GUI) tool that simplifies managing KVM virtual machines. It lets you create VMs, allocate resources, and monitor their status—all through an easy-to-use interface.
Command-Line Tools
KVM can also be fully controlled via the command line. Tools like virsh (for interacting with Libvirt) and virt-install (for creating VMs) offer fine-grained control over every aspect of virtualization.
KVM Installation Guide
Setting up KVM involves checking hardware compatibility, installing the required packages, and configuring network access. Here’s a step-by-step guide:
1. Check Hardware Requirements
Before installation, make sure your system’s CPU supports virtualization technologies like Intel VT-x or AMD-V, and that they are enabled in your BIOS/UEFI.
Run the following command to check:
egrep -c '(svm|vmx)' /proc/cpuinfo
If the result is 0, virtualization is either not supported or disabled in your system’s BIOS/UEFI settings.
2. Install KVM and Related Packages
Installation steps differ depending on your Linux distribution:
For Debian/Ubuntu-based systems:
sudo apt update && sudo apt install qemu-kvm libvirt-daemon-system libvirt-clients bridge-utils virt-manager
For RHEL/CentOS/Fedora systems:
sudo dnf install @virtualization
Or alternatively:
sudo yum install qemu-kvm qemu-img virt-manager libvirt libvirt-python libvirt-client virt-install virt-viewer bridge-utils
3. Enable and Start the Libvirt Service
After installing the packages, start and enable the libvirtd service:
sudo systemctl enable libvirtd sudo systemctl start libvirtd
4. Add Your User to the Libvirt Group
To manage virtual machines without using sudo, add your user to the libvirt group:
sudo usermod -aG libvirt $USER
Then, log out and log back in to apply the change.
5. Configure a Bridge Interface (Optional)Â
If your virtual machines need access to external networks (e.g., the internet), it’s recommended to set up a bridge network.
This involves creating a network bridge configuration file and linking it to your physical network interface. The exact steps vary by distribution and network management tool (e.g., Netplan, NetworkManager, or ifupdown).
Conclusion
In this article, we’ve developed a clear understanding of what KVM is. At its core, KVM is a powerful, open-source virtualization solution that enables the Linux kernel to act as a Type 1 hypervisor. This allows organizations and users to maximize the use of physical hardware by running multiple virtual machines concurrently and efficiently.
With features like zero licensing costs, near-native performance, robust security, proven long-term stability, flexible resource management, and support for Live Migration, KVM presents itself as a strong and attractive alternative to commercial virtualization solutions like VMware.
FAQs
What is KVM and what are its uses?
KVM (Kernel-based Virtual Machine) is an open-source virtualization technology that turns the Linux kernel into a Type 1 hypervisor. Its main use is to create and manage fully isolated virtual machines on a physical Linux server. KVM is ideal for website hosting, application deployment, software development and testing, and building cloud infrastructure.
What are the advantages of KVM?
KVM offers a number of key advantages:
- Free and open-source (no licensing fees)
- High performance, close to bare metal
- Strong security with tools like SELinux and sVirt
- Stability and reliability in production environments
- Flexible resource allocation and management
- Support for Live Migration (moving VMs without downtime)
- Broad compatibility with various hardware and storage systems
Which is better, KVM or VMware?
The choice between KVM and VMware depends on your needs and budget. KVM is a free, open-source solution that offers high performance and flexibility but may require more technical expertise to install and manage. VMware, being a commercial product, offers advanced graphical tools, enterprise-level support, and is widely used in large organizations—but comes with licensing costs.
Is KVM free?
Yes, KVM is completely free and open-source. There are no licensing fees required to use it.
