Virtualization: Part 6 — Xen, XenServer and XCP-ng

Xen — a Type 1 open-source hypervisor: it runs directly on the hardware, before any operating system. The project is developed under the Linux Foundation, the primary license is GPLv2. Current version — Xen 4.22 (30 July 2026), previous — 4.21 (November 2025).

Xen was one of the first widely used open virtualization platforms and for a long time served as the foundation for Amazon Web Services. Today its main areas are ready server platforms XenServer and XCP-ng, systems with enhanced isolation requirements, and embedded systems, including automotive.

How Xen is organized

• Hypervisor — a small layer that allocates CPU and memory between virtual machines. There are almost no hardware drivers in it.
• dom0 — the first, privileged virtual machine (usually Linux). It runs real hardware drivers and management tools; through it other machines get access to disks and network.
• domU — regular guest virtual machines.

This separation is the main architectural difference between Xen and KVM, where the hypervisor is the Linux kernel itself. A small hypervisor is easier to audit, and drivers are placed in a separate machine that can be split into several if needed.

Virtualization modes

• PV (paravirtualization) — the guest OS knows it runs under Xen and calls the hypervisor directly. Historically this provided high performance without hardware virtualization support, but it requires a modified guest kernel and is now used less often.
• HVM — full virtualization with CPU hardware support (Intel VT-x, AMD-V). Allows running unmodified systems, including Windows; for performance guests get paravirtualized disk and network drivers.
• PVH — a modern compromise: hardware virtualization of CPU and memory plus paravirtualized I/O without emulating old devices. This mode is recommended for guest Linux systems; dom0 itself can also run in this mode.

Products based on Xen

XenServer

A commercial platform, previously called Citrix Hypervisor. It is now released by Cloud Software Group under the XenServer name; the current version is XenServer 8. Support for Citrix Hypervisor 8.2 ended on 25 June 2025, so installations on that version need to be upgraded or migrated to another platform.

XCP-ng and Xen Orchestra

XCP-ng is an open platform that grew out of XenServer’s source code. It is developed by the French company Vates. Version XCP-ng 8.3 became a Long Term Support (LTS) release on 16 June 2025; updates for it will be published until 30 November 2028.

XCP-ng is managed through Xen Orchestra — a web application for clusters: creating VMs, live migration, backups, replication, user permissions. XCP-ng 8.3 includes a simplified interface XO Lite for managing a single host. A commercial subscription from Vates provides support and a ready Xen Orchestra build; the source code is open and can be built independently.

For companies moving from VMware, XCP-ng together with Xen Orchestra is one option with a similar model: a pool of hosts, shared storage, live migration, and centralized backups.

Where Xen runs today

• Qubes OS — an operating system for workstations with enhanced security requirements. Each application or group of applications runs in a separate Xen VM, and network drivers and USB are placed in isolated VMs.
• Embedded and automotive systems — Xen on ARM processors allows splitting, on a single device, a functionally safety-critical system and a multimedia system. Much of the work in releases 4.21 and 4.22 is dedicated to this area.
• Clouds. AWS, starting with C5 instances (2017), moved to the Nitro hypervisor based on KVM; older instance types ran on Xen. Most new cloud platforms today are built on KVM.

Xen or KVM

For a small company Proxmox VE is usually simpler: more materials, specialists and ready-made solutions. XCP-ng is chosen when a VMware-like pool model and advanced centralized backups in Xen Orchestra are required.

Common mistakes

• Using Citrix Hypervisor 8.2 after end of support: security updates are not released. You should move to XenServer 8 or XCP-ng.
• Guest Linux in outdated PV mode instead of PVH or HVM with paravirtualized drivers.
• Windows without guest drivers (XenServer VM Tools or XCP-ng drivers): disks and network run in emulation mode and are slow.
• Load on dom0: third-party programs on the management machine consume resources from the drivers that all guest VMs rely on.