Xen maps to Hyper-V as:
Xen microkernel <=> Hyper-V microkernel ==> talks to hardware
Xen dom0 <=> Windows host OS ==> VM that has exclusive access to most hardware on the system, so it gets very near native performance
Xen domU or KVM on top of Xen <=> Hyper-V guest or VBox or VMware guest ==> VM that gets shared access to hardware with things like NAT and virtualized graphics provided by the dom0/host