Virtualization is to provide virtually all types of technologies used on the physical PC. However, in a virtualization technique, services can be modified and adjusted to satisfy user requirements, the cost of hardware purchases and hardware constructions is not occurred. Beside, this virtualization supports multiple guest operating systems on a host OS. It is helpful for virtualization to build cost-effective IT infrastructure. In general, virtualization can be classified into two categories: full virtualization and para-virtualization. Full virtualization does not require a direct interaction with host resources, but in paravirtualization a direct interaction with frequently used resources can substantially improve performance of computation. In this paper, we study the effects on the performance when the VCPUs map to the physical CPUs in paravirtualization. And we study the effects of the mapped VCPU in an overcommit environment that the number of virtual cores is more than the number of physical cores. The purpose of these experiments is to find out the best CPU allocation method in various environments of para-virtualization.