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. In several Grid projects, virtualization tools are envisioned to run many virtual machines per host. This immediately raises the issue of virtualization scalability. In this paper, we compare the scalability merits of Four virtualization tools. First, from a simple experiment, we motivate the need for simple microbenchmarks. Second, we present a set of metrics and related methodologies. We propose four microbenchmarks to measure the different scalability parameters for the different machine resources (CPU, memory disk and network) on three scalability metrics (overhead, linearity and isolation). Third, we compare four virtual machine technologies (Vserver, Xen, UML and VMware). The results of this study demonstrate that all the compared tools present different behaviors with respect to scalability, in terms of overhead, resource occupation and isolation. Thus this work will help user to select tools according to their application characteristics.