from wikipedia "QEMU":

QEMU is a processor emulator that relies on dynamic binary translation to achieve a reasonable speed while being easy to port on new host CPU architectures.

In conjunction with CPU emulation, it also provides a set of device models, allowing it to run a variety of unmodified guest operating systems; it can thus be viewed as a hosted virtual machine monitor. It also provides an accelerated mode for supporting a mixture of binary translation (for kernel code) and native execution (for user code), in the same fashion as VMware Workstation and Microsoft Virtual PC.