xen-hypervisor-4.1-amd64 4.1.6.1-0ubuntu0.12.04.1 (amd64 binary) in ubuntu precise

 The hypervisor is the "core" for XEN itself. It gets booted by the boot
 loader and controls cpu and memory, sharing them between your
 administrative domain (Domain 0) and the virtual guest systems.
 .
 In order to boot a XEN system along with this package you also need a
 kernel specifically crafted to work as the Domain 0, mediating hardware
 access for XEN itself.