From Xen

To allow unprivileged DomUs to share hardware, Dom0 must give them an interface by which to make requests for access to the hardware. This is accomplished by using a BackendDriver. The BackendDriver runs in Dom0 or a DriverDomain and communicates with FrontendDrivers via XenBus, XenStore, and shared memory pages. It queues requests from DomUs and relays them to the real hardware driver.

See also