Performance analysis is a necessary step during the development of distributed applications and communication protocols. Network emulation testbeds provide synthetic, configurable environments for comparative performance measurements of real implementations. However, realistic scenarios require more communicating nodes than usual testbeds are able to provide. In order to enable scalable network emulation, various concepts for the virtualization of nodes have been proposed. The overhead of virtualization strongly impacts the total size of a scenario, that can be emulated on a given testbed. However, the overhead of different virtualization approaches in the context of network emulation has not been compared directly so far. In this paper, we present a comparison of different virtual machine implementations (Xen, User Mode Linux) and our own virtual routing approach (NET). We discuss qualitative evaluation criteria and present a quantitative evaluation showing the efficiency of each approach in a traditional wired infrastructure-based and in a wireless ad hoc network emulation scenario. Our results give insights on which virtualization approach is best suited for which kind of network emulation.