With open source, stable performance, simple structure, integrated in Linux kernel, and good performance, KVM (Kernel-based Virtual Machine) gets wide attention from major IT vendors and academia circles. However, virtualization technology will introduce additional overhead, especially in the I/O virtualization. The current researches about KVM have been carried out mostly focused on the performance comparison and analysis between virtualization solutions. There is no systematic study of the KVM I/O performance. In this paper, the I/O architecture in KVM is firstly studied, and then the disk and network performance of fully virtualization KVM and Para-virtualization KVM with all the different configurations are systemically tested and analyzed. Both the KVM I/O performance and the influences of the different configuration options on the performance are figured out in this paper. These are quite important and useful for the users and designers.