Xen is one of the most popular virtualization platforms nowadays, which has been broadly used by the industry. Credit scheduler, the default scheduler of Xen, was initially designed for serial jobs, which achieves good performance overall for serial jobs. Unfortunately, the parallel jobs are likely to co-exist with serial jobs in the same host in practice, the resource contention between virtual machines results in severe performance degradation of the parallel jobs. In this paper, we propose vGrouper, a progressive solution to enhance the performance of the parallel jobs. The vGrouper focuses on synchronizing the execution time of the parallel nodes in order to achieve the best performance of the parallel job. Moreover, the vGrouper guarantees that the parallel job nodes are able to run concurrently on pCPUs for the entire time slice, which maximizes the efficiency of communication between parallel nodes. A prototype of vGrouper is implemented, the experimental results demonstrate that the performance of the parallel job and resource utilization in Xen have been significantly improved.