Virtualization has become a popular way to make more efficient use of server resources within both private data centers and public cloud platforms. While recent advances in CPU architectures and new virtualization techniques have reduced the performance cost of using virtualization, overheads still exist, particularly when multiple virtual machines are competing for resources. We have performed an extensive performance comparison under hardware-assisted virtualization settings considering four popular virtualization platforms, Hyper-V, KVM, vSphere and Xen, and find that the overheads incurred by each hypervisor can vary significantly depending on the type of application and the resources assigned to it. We also find dramatic differences in the performance isolation provided by different hypervisors. However, we find no single hypervisor always outperforms the others. This suggests that effectively managing hypervisor diversity in order to match applications to the best platform is an important, yet unstudied, challenge. © 2013 IFIP.