QEMU compared to other emulators¶
Like bochs , QEMU emulates an x86 CPU. But QEMU is much faster than bochs as it uses dynamic compilation. Bochs is closely tied to x86 PC emulation while QEMU can emulate several processors.
Like Valgrind , QEMU does user space emulation and dynamic translation. Valgrind is mainly a memory debugger while QEMU has no support for it (QEMU could be used to detect out of bound memory accesses as Valgrind, but it has no support to track uninitialised data as Valgrind does). The Valgrind dynamic translator generates better code than QEMU (in particular it does register allocation) but it is closely tied to an x86 host and target and has no support for precise exceptions and system emulation.
EM86  is the closest project to user space QEMU (and QEMU still uses some of its code, in particular the ELF file loader). EM86 was limited to an alpha host and used a proprietary and slow interpreter (the interpreter part of the FX!32 Digital Win32 code translator ).
TWIN from Willows Software was a Windows API emulator like Wine. It is less accurate than Wine but includes a protected mode x86 interpreter to launch x86 Windows executables. Such an approach has greater potential because most of the Windows API is executed natively but it is far more difficult to develop because all the data structures and function parameters exchanged between the API and the x86 code must be converted.
User mode Linux  was the only solution before QEMU to launch a Linux kernel as a process while not needing any host kernel patches. However, user mode Linux requires heavy kernel patches while QEMU accepts unpatched Linux kernels. The price to pay is that QEMU is slower.
The Plex86  PC virtualizer is done in the same spirit as the now obsolete qemu-fast system emulator. It requires a patched Linux kernel to work (you cannot launch the same kernel on your PC), but the patches are really small. As it is a PC virtualizer (no emulation is done except for some privileged instructions), it has the potential of being faster than QEMU. The downside is that a complicated (and potentially unsafe) host kernel patch is needed.
The commercial PC Virtualizers (VMWare , VirtualPC ) are faster than QEMU (without virtualization), but they all need specific, proprietary and potentially unsafe host drivers. Moreover, they are unable to provide cycle exact simulation as an emulator can.
VirtualBox , Xen  and KVM  are based on QEMU. QEMU-SystemC  uses QEMU to simulate a system where some hardware devices are developed in SystemC.