What Does Xen Do?
Unlike many other programs, Xen tries to do as little as possible. Successive versions do less and less, with more functionality being delegated back to the host operating system—domain 0 in Xen parlance.
A combination of Xen and the domain 0 operating system provides the virtualization host. On top of this other hosts are run in virtual containers.
Xen does not provide device drivers itself; it routes all hardware accesses via the domain 0 host.