Cloud computing is an emerging paradigm where computing services are provided across the web. Virtualization powers the cloud by mutualizing physical resources thus ensuring flexibility and high availability of the cloud. Certainly fault tolerance like load balancing or advancement programming security aim to foster availability but classic reactive fault tolerance techniques prove to be greedy in terms of memory and recovery time. Elsewhere, proactive fault tolerance is possible by preemptive virtual machine migration requiring a strong and accurate failure predictor. In quest of an effective approach for proactive fault tolerance we introduce in this paper a probabilistic model of the cloud with a failure generator for evaluating a proposed approach based on three scenarios of virtual machine migration.