In the HVM case it is a little bit like this (simplified):
This is the simple case of a HVM (hardware virtualized machine), needed to run - for example - Windows. The emulated IDE controller with its virtual IDE disk (named "hda" in Xen) is normally not used (it needs the "qemu-dm" daemon on the Dom0 side which "offers" big performance losses). After having installed Xen drivers the "xdv*"-Devices are used - directly connected to the Xen-Bus present in every DomU virtual machine.
All storage access - if no controller is directly passed through the Hypervisor exclusively - is done via Domain-0 (Dom0).
In a paravirtualized Dom0 it is even simplier:
The pictures illustrate the Sun xVM case. In a Linux Xen system it is pretty the same but access is not done via ZFS block volumes but normally via Image files on an ext3 volume in Dom0.