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 vir­tu­al­iza­tion 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 vir­tu­al­iza­tion 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 par­avir­tu­al­iza­tion to boost ef­fi­cien­cy

Xen uses the vir­tu­al­iza­tion technique par­avir­tu­al­iza­tion to boost ef­fi­cien­cy. With this technique, the hy­per­vi­sor doesn’t emulate hardware com­po­nents, and guest systems are aware that they are operating in a vir­tu­al­ized 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 par­avir­tu­al­iza­tion for years.

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

KVM has since been expanded to include par­avir­tu­al­iza­tion. 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 vir­tu­al­ized systems and their processes.

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

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

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 vir­tu­al­iza­tion to vir­tu­al­ize numerous hardware com­po­nents without ex­pe­ri­enc­ing sig­nif­i­cant per­for­mance drawbacks. The primary IO in­ter­faces are par­avir­tu­al­ized, 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 vir­tu­al­iza­tion 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 vir­tu­al­ized 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 par­avir­tu­al­ized network in­ter­faces, each system vir­tu­al­ized 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 vir­tu­al­iza­tion. These aspects make it fun­da­men­tal­ly easier to use.

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