Virtualization is the process of running several Operating Systems on one piece of hardware. It is a good solution to save money on hardware and can also reduce the power consumption considerably. Virtualization becomes more popular each day and a lot of organizations utilize virtualization to run as many of their servers as they can on the same machine. Because of this it is interesting to look at the security of the applied virtualization solutions for vulnerabilities.
A hypervisor is the interface between the hardware and the virtual machines (VMs) and it is up to the hypervisor to distribute the available resources to the VMs in such a way that they can function properly for their specific purpose. For reference, a Virtual Machine Monitor (VMM) is the same as a hypervisor.
Organizations that are utilizing virtualization to a great extent are much more vulnerable than those who are not. Care must be taken when stuffing as many servers as possible on one machine without considering the implications of what might happen if an exploitable vulnerability is found in the hypervisor.