Virtualization is essential in supporting today's information infrastructure, and in particular the Cloud Computing area. However, the move to a virtualized architecture implies the addition of a new single point of failure: the hypervisor. Attempts to characterize and compare the susceptibility of systems (including virtualized systems) are often limited to the study of failure modes and their probabilities. Although undoubtedly useful, in isolation it is not enough to accurately depict the susceptibility of a system, and much less to enable comparison. In this paper, the failure mode analysis of a new and promising virtualization mode of the leading hypervisor in cloud computing deployments (Xen) is performed, followed by the presentation of a general approach to evaluate and compare the susceptibility of systems to soft errors. Exemplifying the approach, a comparison between the susceptibility of three virtualization modes (PVH, HVM and PV), for soft errors in processor registers that occur in a privileged virtual machine (Domain-0), ensues.