With the resurgence of virtual machine technology, vendors build virtualization technology support in the processors. Virtual machines relying on such support are called Hardware-assisted Virtual Machines (HVM guests for short). Although HVM guests have the advantage that they do not need to revise the source code of operating systems running inside, the processing of I/O operations in HVM is much complicated and the performance of such operations are rather low. In this paper, we conduct a performance study on the Web server running inside the HVM guests by using queuing network modeling technique, and analyze the performance bottlenecks of the systems.