In the virtualization area, replication has been considered as a mechanism to provide high availability. A high-availability system should be active most of the time, and this is the reason that its design should consider almost zero downtime and a minimal human intervention if a recovery process is demanded. Several migration and replication mechanisms have been developed to provide high availability inside virtualized environments. In this article, a survey of migration mechanisms is reported. These approaches are classified in three main classes: process migration, memory migration, and suspend/resume migration.