QEMU is widely used in Cloud environments, it powers both Xen and KVM. Basically it is a fast and portable dynamic translator and an embedded machine emulator that emulates multiple CPUs and many board models. It can also provide a virtual platform for quickly software development such as Android Emulator, which uses it to emulate the whole mobile platform. But QEMU only supports common hardware, so new virtual device module should be developed for QEMU in order to emulate new hardware. In this paper, we finish our research on full system emulation, study the architecture and internals of QEMU, propose detailed steps to create user-defined virtual hardware devices and also develop the Linux kernel driver for the new device. Research results show that the whole running, testing and debugging environment are built, and user level applications can be developed for the new virtual hardware without the physical device become available.