Virtualization tools are becoming popular in the context of Grid Computing because they allow running multiple operating systems on a single host and provide a confined execution environment. Emulators and experimental tools like Microgrid and PlanetLab use multiple virtual nodes on every platedorm nodes. Thus, to build a large scale emulator for Grid/P2P experiments, developers have to select a virtualization tool among the available ones. In this paper, we first present a set of metrics and related methodologies for performances and for usability evaluation. We use thesemetrics to compare 4 virtual machine technologies (Vserver, Xen, UML and VMware), in the context of large scale emulation. The results of this study demonstrate that all the compared tools present different behaviors with respect to usability, and scalability in terms of overhead, resource occupation and isolation. Thus this work will help user to select tools according to their application characteristics.