Virtualization is a technique and methodology used to provide a certain type of hardware and software environment, specifically, one that is a complete simulation of the underlying hardware and software. Virtualization requires that every salient feature of the hardware and software be reflected into one of several  virtual machines– including the full instruction set, input/output operations, interrupts, memory access, and whatever other elements are used by the software that runs on the bare machine, and that is intended to run in a virtual machine.

 In such an environment, any software capable of execution on the raw hardware can be run in the virtual machine and, in particular, any operating systems. The obvious test of virtualization is whether an operating system intended for stand-alone use can successfully run inside a virtual machine.

Other forms of virtualization allow only certain or modified software to run within a virtual machine. A key challenge of virtualization is the interception and simulation of privileged operations, such as I/O instructions. The effects of every operation performed within a given virtual machine must be kept within that virtual machine – virtual operations cannot be allowed to alter the state of any other virtual machine, the control program, or the hardware. Some machine instructions can be executed directly by the hardware, since their effects are entirely contained within the elements managed by the control program, such as memory locations and arithmetic registers. But other instructions that would "pierce the virtual machine" cannot be allowed to execute directly; they must instead be trapped and simulated. Such instructions either access or affect state information that is outside the virtual machine.

Full virtualization has proven highly successful for:

1. Sharing a computer system among multiple users,
   
2. Isolating users from each other (and from the control program) and
   
3. Emulating new hardware to achieve improved reliability, security and productivity
   Its full use in standing up, consolidation and cost reduction in data centers therefore become obvious on examination.