Xen or KVM: Open-Source Hypervisors Compared

When it comes to open-source virtualisation and the question of whether to use KVM or Xen as a hypervisor, the answer may seem clear at first glance. KVM appears to have taken the lead in what was once a close race, especially since it became an integral part of Red Hat Linux. However, the question is not entirely settled, as Xen should not be disregarded in open-source virtualisation, as it also offers its own advantages. Let's take a closer look at the topic.

The Rise of KVM

It is fair to say that KVM has established itself as the hypervisor for open-source virtualisation in the market. While it may be considered the quasi-standard now, it is not accurate to say that Xen no longer has a role to play or any advantages to offer.

Key Differences and Potential Advantages of Xen

One fundamental difference with Xen is that it is undeniably a Type-1 hypervisor. This distinction is not as clear with KVM, as even experts disagree on the type of hypervisor KVM is. We have already covered more about hypervisor types and the fundamental differences in server virtualisation in our guide.

This single, fundamental difference leads to significant consequences. Xen, as a Type-1 hypervisor, efficiently allocates server resources to guest systems using paravirtualisation, allowing device drivers installed in the VM direct access to the hardware. This results in significantly better performance compared to other systems. While this may not seem crucial in today's era of abundant server resources (things were quite different in the early 2000s during Xen's heyday), better performance and more efficient use of server resources can never hurt in open-source virtualisation.

Also, learn about the difference compared to container virtualisation, as in the case of LXC vs KVM.

To what extent this affects performance in practice can only be tested on an individual system: Theoretically, performance is better, but whether this translates to your own server in practice needs to be tested.

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Furthermore, if a virtual machine goes rogue under KVM, it almost inevitably affects the entire server and usually all other VMs, as the host kernel always sees the entire hardware. With Xen, such a scenario is simply impossible, as the hypervisor manages the hardware. Problems with a VM always remain confined to that specific VM. Depending on the use case for the open-source virtualisation being used, this can be an important point in individual cases.

When "passing through" the server hardware to the VMs, Xen is more thorough, but the advantage is not as significant anymore, as both systems handle this quite well. Significant differences in this aspect of open-source virtualisation are unlikely to be found in either system.

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KVM vs Xen: Conclusion

KVM is the quasi-standard, no question. However, Xen still emerges as a good and often advantageous alternative, equally future-proof and now with almost as little effort to install and set up. Whether the potential benefits of Xen will have a corresponding impact on your own server can only be determined on a case-by-case basis.

Testing is recommended nonetheless - and Xen should definitely be considered as a valid alternative to KVM.

Our article is based on our own experiences and research, as well as information from external sources.

References & further links on the topic:
https://www.linux-kvm.org/page/Main_Page (Official Wiki of the KVM community)
https://www.redhat.com/de/topics/virtualization/what-is-KVM (Information from the owner of the KVM developer)
https://xenproject.org/ (Official website of the Xen project)
https://www.linux-magazin.de/ausgaben/2017/12/xen/ (Article on Xen in Linux Magazine)

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