http://technet.microsoft.com/en-us/edge/Feature-of-the-Week-Hyper-V-Server.aspx

"Loads a driver set and base OS that you host virtual machines on. No UI, no extra services. It only hosts VMs that you manage from another machine, either using the Hyper-V Management console or Virtual Machine Manager 2008."

Wouldn't Hyper-V Server have significantly less services and therefore less holes that need to be patched? I've not used Hyper-V so I can't say for sure. I'm curious to know if you've used Hyper-V and if it's required updates every month or if you're basing your statements on a full blown Windows install?