DomU

DomU, short for Domain U, is the unprivileged guest domain in the Xen virtualization platform. It runs user workloads and is isolated from the privileged Dom0, which manages hardware access and the Xen toolstack.

In Xen, the hypervisor runs directly on hardware. Dom0 owns hardware, presents virtual devices to DomU, and handles I/O through a backend/frontend driver model. DomU instances can be paravirtualized (PV) with modified guest OS drivers or hardware-assisted (HVM) using CPU virtualization extensions. Some newer systems use PVH, a hybrid PV/HVM guest design for better performance and simplicity.

Creation and management: DomU are created and managed by the Xen toolstack (examples: xl, libxl, or XAPI in XenServer/OpenStack deployments). They boot from kernel/initramfs and disk images, and are allocated CPU, memory, and I/O resources. Network and storage access is provided via virtual network interfaces and disk backends in Dom0.

Operations: DomU are isolated from one another; the hypervisor enforces memory and CPU quotas. Live migration can move DomU between physical hosts with shared storage. DomU can be paused or suspended; snapshots or backups may be performed depending on tooling.

Usage: DomU are widely used to run guest operating systems in Xen-based environments, including data centers, private clouds, and research environments. They enable strong isolation and flexible resource management while letting Dom0 handle hardware access and management.