Cloud computing has become an important infrastructure of the Internet, and more and more applications are deployed into cloud computing to provide the online service. Virtualization technology is the key element of cloud computing and hot research field, and it provides the computation, storage and network resource virtualization. KVM whose kernel is virtualization has become the one of the main virtualization technologies. This paper analyzes the structure of KVM, and introduces the working process of KVM. Finally, we also test the performance of KVM and get the useful results. These results, which are useful for technology improvement, can be used to improve the virtualization technology.