As enterprises move towards virtualizing more of their servers and data center infrastructure, the security technologies that are plentiful and commonplace in the physical world become few and far between.
While few direct attacks on virtual machines have been observed, it is still good security practice to protect VMs from potential vulnerabilities that exist only in the virtualized world.
For example, physical firewalls aren't designed to inspect and filter the vast amount of traffic originating from a hypervisor running 10 virtualized servers. And because VMs can start, stop, and move from hypervisor to hypervisor at the click of a button, protective features have to be able to handle these movements and activities with ease. Finally, few hypervisors have the access controls that even the most basic file server has: once someone can gain access to the hypervisor, they can control all of the VMs that are housed there.