There are several laws of information security. Ask ten InfoSec pros and you will likely get ten different lists of laws, but I wager every one of them will agree on a couple fundamentals. If an attacker can gain physical access to the computer, or if an attacker can modify the operating system, then the attacker can compromise the computer. The reason is physical access allows an attacker to bypass the OS and directly access the data, and bypass the security controls.
Now, switch gears and picture a virtual environment. The physical analog is the hypervisor. If an attacker can gain access to the hypervisor, he has the same abilities as if he had access to the physical computer. If an attacker can exploit the Windows or Linux server hosting Hyper-V or XenServer, then the attacker can compromise all virtual computers on the host.
It is a subtle shift in the way of thinking. In the past, only one server ran on one piece of hardware, and the security boundary was the server itself. Thus you would place a physical web server in the DMZ and physically wire it to the firewalls. Computers with different security postures (e.g., domain controllers) would be on separate physical hardware and wired into separate physical networks.
Thus the hypervisor should host servers that have relatively the same security posture. One should not, for instance, host domain controllers and public-facing web servers on the same hypervisor. Even if the public-facing web server is on a separate virtual network, you still run the risk of its compromise affecting the domain controllers.
The security boundary is the physical hardware, not the computer itself.