There has been an increasing interest in virtualization in the HPC community, as it would allow to easily and efficiently share computing resources between users, and provide a simple solution for checkpointing. However, virtualization raises a number of interesting questions, on performance and overhead, of course, but also on the fairness of the sharing. In this work, we evaluate the suitability of KVM virtual machines in this context, by comparing them with solutions based on Xen. We also outline areas where improvements are needed, to provide directions for future works.