The Xen hypervisor is one of the oldest open-source type 1 hypervisors. It powered the early public clouds and is still used in XCP-ng, Citrix Hypervisor and security-focused systems such as Qubes OS. This guide explains its architecture and guest modes.
Before you start
- Know the basics of type 1 hypervisors and paravirtualization.
Xen architecture
Xen is a small microkernel-style hypervisor that boots first and schedules CPU and memory. It does not contain device drivers itself.
- Dom0: a privileged Linux VM that boots right after Xen. It has the hardware drivers, runs the toolstack (
xl, XAPI) and provides virtual disks and networks to guests. - DomU: unprivileged guest VMs, which is where customer workloads run.
- Driver domains (optional): separate VMs for network or storage drivers to limit the impact of a driver bug.
Xen guest modes
| Mode | How it works | Notes |
|---|---|---|
| PV | Modified kernel, hypercalls, no VT-x needed | Legacy; weaker against some CPU side-channel attacks |
| HVM | Hardware-assisted full virtualization with QEMU device emulation, plus PV drivers | Runs Windows and unmodified OSes |
| PVH | Hardware assistance for CPU/memory, PV interfaces for devices, no QEMU | Modern recommended mode for Linux guests |
Xen platforms
- XCP-ng: free, open-source Xen distribution with the XAPI toolstack; manage with Xen Orchestra.
- Citrix Hypervisor (XenServer): commercial distribution.
- Plain Xen on Debian or similar: managed with
xlor libvirt.
Xen vs KVM
| Xen | KVM | |
|---|---|---|
| Design | Separate hypervisor + Dom0 | Part of the Linux kernel |
| Ecosystem | XCP-ng, Xen Orchestra | Proxmox, OpenStack, most VPS panels |
| Isolation focus | Strong, small trusted core | Strong, uses Linux security features |
| VPS market share today | Smaller | Dominant |
Frequently asked questions
Is Xen still maintained?
Yes. The Xen Project under the Linux Foundation continues active development, and XCP-ng has a growing community.
Can I tell if my VPS runs on Xen?
Run systemd-detect-virt; it prints xen on Xen guests.
Should I pick Xen or KVM for a new VPS platform?
Most VPS panels and tooling target KVM, so KVM is usually the easier choice unless you specifically want XCP-ng.