There is a general paranoia about server virtualization in the security community that goes something like this. The server virtualization hypervisor acts as a resource switch enabling multiple virtual hosts to share a single physical system. In theory, if you compromise the hypervisor, you gain access to every virtual host along for the ride. Imagine an instance where 50 hosts live on a single Intel server and you can see that a hypervisor attack could have extremely serious ramifications.

Yes, this is theoretically possible, but virtualization vendors understand this threat and are pretty conscientious about protection. [..] So what is it about server virtualization that should really keep chief information security officers up at night? A more pedestrian worry--lack of control. In a virtual server world, IT administrators can clone virtual hosts, move them around, or turn them on and off by accident or with malicious intent. What happens when an IT administrator moves a critical database server instance without re-configuring application servers or the network?  How about when someone mistakenly adds a test server to the production network? The security "uh-oh" possibilities are endless.