History
Paravirtualization is a new term for an old idea. IBM's VM operating system has offered such a facility since 1972 (and earlier as CP-67). In the VM world, this is referred to as a "DIAGNOSE code", because it uses an instruction code used normally only by hardware maintenance software and thus undefined.
The Parallels Workstation operating system calls its equivalent a "hypercall". All are the same thing: a system call to the hypervisor below. Such calls require support in the "guest" operating system, which has to have hypervisor-specific code to make such calls.
The term "paravirtualization" was first used in the research literature in association with the Denali Virtual Machine Manager. The term is also used to describe the Xen, L4, TRANGO, VMware, Wind River and XtratuM hypervisors. All these projects use or can use paravirtualization techniques to support high performance virtual machines on x86 hardware by implementing a virtual machine that does not implement the hard-to-virtualize parts of the actual x86 instruction set.
A hypervisor provides the virtualization abstraction of the underlying computer system. In full virtualization, a guest operating system runs unmodified on a hypervisor. However, improved performance and efficiency is achieved by having the guest operating system communicate with the hypervisor. By allowing the guest operating system to indicate its intent to the hypervisor, each can cooperate to obtain better performance when running in a virtual machine. This type of communication is referred to as paravirtualization.
In 2005, VMware proposed a paravirtualization interface, the Virtual Machine Interface (VMI), as a communication mechanism between the guest operating system and the hypervisor. This interface enabled transparent paravirtualization in which a single binary version of the operating system can run either on native hardware or on a hypervisor in paravirtualized mode. In September 2009, VMWare announced that VMI would be retired from future products.
Read more about this topic: Paravirtualization
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