Here is a link to the Windows 2008 R2 Hyper-V Security updates. (http://technet.microsoft.com/en-us/library/ff394763(WS.10).aspx)
There are over 4 critical BSOD (Blue Screen of Death) and more critical software updates (25+) for just this edition of Hyper-V and countless support forums problems/fixes.
There are even more Security and Critical updates required for Windows Servers on top of the Hyper-V updates.
VMware 4.1.0 has 36 + updates since its release date :- ( http://www.vmware.com/patch/download/)
There are even more updates for vCenter and also you will need updates for your SQL DataStore Server
You may need updates for the following Microsoft Management tools as well :-
System Center Management Suite Enterprise , System Center Virtual Machine Manager, System Center Ops Manager Server, System Center Configuration Manager, System Center Data Protection Manager
XenServer 5.6 FP1 hotfixes link – (http://support.citrix.com/product/xens/v5.6fp1/) (ONE Hotfix for Neterion Inc.’s X3100 Series 10GbE PCIe I/O Virtualized Server Adapter)