Paravirtualization
In paravirtualization, the virtual machine does not necessarily simulate hardware, but instead (or in addition) offers a special API that can only be used by modifying the "guest" OS. This system call to the hypervisor is called a "hypercall" in TRANGO and Xen; it is implemented via a DIAG ("diagnose") hardware instruction in IBM's CMS under VM (which was the origin of the term hypervisor). Examples include IBM's LPARs, Win4Lin 9x, Sun's Logical Domains, z/VM, and TRANGO.
Read more about this topic: Hardware Virtualization