Xen is a virtual-machine monitor providing services that allow multiple computer operating systems to execute on the same computer hardware concurrently.
In Xen systems the Xen hypervisor is the lowest and most privileged software layer. This layer supports one or more guest operating systems, scheduled on the physical CPUs. The first guest operating system, called in Xen terminology domain 0 (dom0) is executed automatically when the hypervisor boots and receives special management privileges and direct access to all physical hardware by default. The system administrator can log into dom0 in order to manage any additional guest operating systems, called user domains (domU) in Xen terminology.