Xen consists of the following two major components:
The virtual machine monitor forms a layer between physical hardware and virtual machines. In general this kind of software is called a Hypervisor.
The Xen tools are a set of command line applications that are used to administer virtual machines.
The virtual machine monitor must be loaded before any of the virtual machines are started. When working with Xen, virtual machines are called domains.
The Xen virtual machine monitor neither includes any drivers to access the physical hardware of the host machine nor an interface to communicate directly with an administrator. These tasks are performed by an operating system running in the privileged domain0.
A process called xend runs in the domain0 Linux installation. This process is used to manage all Xen domains running on a system and to provide access to their consoles.
A unprivileged domain is also called domainU in the Xen terminology.