Paper: Virtual Machine Threats
Virtual Machine Threats by Symantec Research
As virtual machine emulators have become commonplace in the analysis of malicious code, malicious code has started to fight back. This paper describes known attacks against the most widely used virtual machine emulators (VMware and VirtualPC). This paper also demonstrates newly discovered attacks on other virtual machine emulators (Bochs, Hydra, QEMU, and Xen), and describes how to defend against them.
I highly recommend you read this:
Its a very comprehensive paper on Virtual Machines and their issues. As a bonus they cite an Offensive Computing paper in the references :)