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→Hypervisor Virtualization
The x86 family of CPUs provide a range of ''protection levels'' also known as ''rings'' in which code can execute. Ring 0 has the highest level privilege and it is in this ring in that the the operating system kernel runs. Code executing in ring 0 is said to running in ''system space'', ''kernel mode'' or ''supervisor mode''. All other code such as applications running on the operating system operate in less privileged rings, typically ring 3.
Under hypervisor virtualization a program known as a ''hypervisor '' (also known as a type 1 Virtual Machine Monitor or VMM) runs directly on the hardware of the host system in ring 0. The task of this hypervisor is to handle resource and memory allocation for the virtual machines in addition to administration and monitoring interfaces.
Clearly, with the hypervisor occupying ring 0 of the CPU, the kernels for any guest operating systems running on the system must run in unprivileged CPU rings. Unfortunately, most operating system kernels are written to run in ring 0 for the simple reason that they need to perform tasks that are only available in that ring such as the ability to execute privileged CPU instructions and directly manipulate memory. A number of different solutions to this problem have been devised in recent years, each of which is described below:
