Xen vs. KVM, the hypervisors compared

In the age of cloud computing, hypervisors like KVM and Xen have become more crucial than ever. But how do these two virtualisation technologies differ? With our Xen vs. KVM comparison, we’ll take a look at the similarities and differences between the two.

Xen at a glance

Xen is open-source virtualisation software that makes it possible to simultaneously run multiple guest operating systems on a single host machine. Xen manages the allocation of the hardware resources on the host machine.

What type of hypervisor is Xen?

Xen is a Type 1 (or bare metal) hypervisor. This type of hypervisor is installed directly onto a physical system and communicates with hardware through its own drivers. Host and guest operating systems are installed above the Xen hypervisor. To differentiate between different levels of privilege, Xen categorizes the operating systems that are installed into two different domains: dom0 and domU. The host operating system belongs to the dom0 domain and enjoys the highest level of privilege, while all guest operating systems are classified as unprivileged systems and belong to the domU domain.

The host operating system doesn’t communicate directly with the hardware through its drivers but instead through the hypervisor using a specialized Xen API. This deep integration ensures more efficient use of the hardware, less overhead and a reduced attack surface.

How Xen uses paravirtualisation to boost efficiency

Xen uses the virtualisation technique paravirtualisation to boost efficiency. With this technique, the hypervisor doesn’t emulate hardware components, and guest systems are aware that they are operating in a virtualised environment. This allows for much faster communication between guest operating systems and Xen, leading to increased performance.

However, this approach relies on specialized drivers and specific built-in capabilities in the operating system kernel. Many widely used operating system kernels such as Linux and BSD have been able to support paravirtualisation for years.

If you are using an operating system where paravirtualisation is not possible or not desired, Xen also supports full virtualisation (where hardware components are virtually emulated). Although this slows down virtualisation, it provides high compatibility, allowing operating systems that do not support paravirtualisation to run under Xen.

What are KVM’s key features?

Like Xen, KVM (Kernel-based Virtual Machine) is also an open-source hypervisor that lets users run multiple guest operating systems on a single physical machine. While both hypervisors serve the same fundamental purpose, their architectural design is different, with each hypervisor being suitable for different use cases.

What type of hypervisor is KVM?

In the past, KVM was often referred to as a Type 2 hypervisor, because it could only communicate with hardware through the drivers of the host operating system. KVM had to emulate all hardware components for the guest systems, making paravirtualisation impossible.

KVM has since been expanded to include paravirtualisation. The way that it operates is different to Xen though. Unlike Xen, KVM is not installed under the host operating system but instead directly integrated into the Linux kernel. KVM uses the software components of the Linux system to manage virtualised systems and their processes.

Under KVM, each guest operating system has its own virtualised set of hardware, including network interfaces, disk storage and other types of hardware components. To accelerate full virtualisation, KVM employs hardware-assisted virtualisation, which is already present in most modern processors.

Not every component undergoes full virtualisation in KVM. To enhance the performance of running systems and applications, KVM provides paravirtualised interfaces that can directly communicate with the hypervisor through the Virtio API. KVM predominantly uses paravirtualisation for input and output devices like the network interface. That’s why when discussing KVM, people often describe it as employing partial paravirtualisation.

KVM vs. Xen, how their performance compares

The main difference between the two hypervisors lies in where they operate within the software stack. KVM is integrated into the Linux kernel and uses hardware-assisted virtualisation to virtualise numerous hardware components without experiencing significant performance drawbacks. The primary IO interfaces are paravirtualised, which, for most server applications, is probably the most crucial aspect.

Xen, on the other hand, is traditionally installed under the operating system as a Type 1 hypervisor. This means that the hypervisor manages the distribution of hardware resources itself. Theoretically, this allows for Xen to provide more efficient and performant communication with the hardware. However, KVM’s Virtio drivers and hardware-assisted virtualisation put its level of performance on par with Xen.

CPU pinning

Despite exhibiting similarities in performance levels, the two hypervisors do not perform equally in all scenarios. When it comes to CPU pinning, KVM offers more power to its users. With CPU pinning, KVM can assign a physical processor (or a processor core) to a virtualised processor, ensuring exclusive usage for that virtual system. Of the two hypervisors, only KVM provides this feature, which is particularly beneficial for CPU-intensive applications.

Network performance

In terms of network performance, Xen takes the lead. Even though both hypervisors provide paravirtualised network interfaces, each system virtualised under KVM has its own network device. With Xen, all guest systems share a single virtual network interface, which leads to increased efficiency.

Summary

The question of which hypervisor is better depends on your requirements. The market leader KVM is integrated into the Linux kernel and offers the possibility of full virtualisation. These aspects make it fundamentally easier to use.

However, Xen’s full paravirtualisation capabilities means that it can theoretically provide better performance. The crucial factors you should consider when choosing a hypervisor are the applications you want to virtualise and the level of experience you have.