XenServer, which is built on open source Xen, takes a slightly different approach. Recognizing the need for different layers that need different privileges for accessing the underlying resources, XenServer uses the concept of Dom0 and DomU, and with the release of XenServer v6.5, Dom0 is now a 64-bit architecture. The following is a detailed description of them:
Dom0 stands for Domain 0 and is the privileged space that provides access to disks, networks, memory, and CPUs. Dom0 contains all the necessary drivers to provide access to the virtual machines. Dom0 is similar to kernel space.
Virtual machines and applications run in DomU, which stands for Domain User or User Domain. Providing access to the underlying hardware in Xen is done through backend drivers in Dom0 that communicate with frontend drivers in DomU. DomU is where your virtual machines run. In terms of server architectures, DomU is analogous to user space. You can have multiple DomUs running on top of Dom0: