A server is a server, right? They are just made up of hardware and software. But virtual servers have different hardware and software and and bring different and very real security risks. Let's look at the risks so we can understand how virtual servers are different:
This does not tell the whole story. The Burton Group says that Hyper-V falls short on must-have features. Specifically, Hyper-V lacks the ability to restart VMs in a specific order. This could be a security problem if one server needs to be up before another. In addition, Hyper-V falls short supporting multiple CPUs on Windows Server 2003 and earlier versions of the OS. Finally, Microsoft's Virtual Machine Manager cannot run on a cluster of servers so it can't be made truly fault tolerant. Xen Server didn't do all that much better. The only VM to meet all of the 27 essential features was EMC's VMware.
In my opinion, Microsoft and Xen are just trying to get the basic VM to work properly. EMC is clearly the leader and surpasses them both. EMC has recently released VMSafe, a set of security APIs that addresses security, is included in VMCenter, and can be extended across the VM line. VMSafe runs at the Hypervisor layer and provides monitoring of VMs, enforces policies, and acts as a malicious activity scanner. What VM is your organization using?