Combining end-host, server and router virtualization could offer isolated and malleable virtual networks of different types, owners and protocols, all sharing one physical infrastructure. However, the virtualization of data plane may lead to performance degradation and indeterminism. These arise not only due to additional processing, but also from the sharing of physical resources like memory, CPU and network devices. This paper analyses virtualization from the data plane perspective. We explore the resulting network performance in terms of throughput, packet loss and latency between virtual machines, and also the correspondig CPU cost. The virtual machines act as senders or receivers, or as software routers forwarding traffic between two interfaces in the context of Xen. Our results show that the impact of virtualization on network performance is getting smaller with the successive Xen versions, making this approach a promising solution for data plane virtualization.