With the release of Citrix XenServer 5.6 Citrix also released a new virtual appliance dedicated to licensing, the License Server VPX. The appliance is available since July 2010, in the 2.5 years the appliance is available only Citrix XenServer is supported.
If you’re company is using VMware vSphere (or Microsoft Hyper-V) you’re unable to use the virtual appliance. Citrix refuses to release an appliance for other hypervisors then their own, let alone supporting other hypervisors. Technically it shouldn’t be too difficult to support other hypervisors (some people tried), they already do for the NetScaler VPX.