Operating systems provide both mechanism and policy. Policy is how to use the mechanism in specific situations

Server virtualization places a software layer called a hypervisor (e.g., virtual machine monitor or VMM) between a machine (e.g., server) hardware and the operating systems that run on it An operating system (OS) is at the core of all of the connected hardware and software

policy — controls how to use a mechanism in specific situations, that is, choose what activities need to be done. operating system (OS) — core piece of software that typically manages the interconnection of hardware and software on a given computer. virtualization — allows a system to run different types of applications used by multiple users at a time on the same computer. mechanism — activities that enforce policies and often depend on the hardware on which the operating system runs.