Virtual Machine (VM) technology is experiencing a resurgent interest as the ubiquitous multi-core processors have become the de facto configuration on modern Web servers. Multicore servers potentially provide sufficient physical resources to realize VM's benefits including performance isolation, manageability and scalability. However, the network performance of virtualized multi-core servers falls short of expectation. It is therefore important to understand the overhead implications. In this paper, we evaluate the network performance of a virtualized multi-core server using a TCP streaming microbenchmark (Iperf) and SPECweb2005. We first motivate our research by presenting the performance gap between native and virtualized environment. We then break down the overhead from an architectural viewpoint and show that the cache topology greatly influences the performance. We also profile the virtual machine monitor (VMM) at a function level to illustrate that functions in the current version of the Xen scheduler are the major contributors to the poor utilization of cache topology. Consequently, we implement a static onloading scheme to separate interrupt handling from application processes and execute them on cores with cache affinity. Based on the observed benefits, we modify the Xen scheduler to migrate virtual CPUs dynamically to exploit the cache topology. Our results show that the VM performance improves by an average of 12% for Iperf and 15% for SPECweb2005.