VMWare is a commercial VirtualMachine that emulates an entire PC in a PC.

VMWare is special in that it makes an unusual assumption to allow for an unusual optimization that allows for unusual performance of emulated code: VMWare requires a PC to run. Instead of emulating the CPU along with the rest of the hardware by interpreting every single MachineCode instruction running in emulation, it runs the code non-emulated on the real hardware CPU, except for any instructions used by the OperatingSystem or hardware drivers to interact with the machine. Those are trapped and interpreted as required to set up the VirtualMachine smoke and mirrors. The result is that code in emulation runs blazingly fast compared to any competitor. The speed of applications that do not perform much I/O may approach that of running unemulated.

Applications that perform a lot of bulk I/O may even run faster, because they do not require a lot of emulation and can benefit from Caches at the underlying OperatingSystem's level. Inbetween those extremes, emulation is as costly as ever.