This paper explores the use of hypervisor-based virtualization technology as a means to enable power management in server systems. Our approach focuses on the dynamic mapping of physical processors and hosts to virtual machines. We have developed a multi-tiered infrastructure that enables dynamic migration of virtual machine execution flow at two different levels: within and across computer nodes. Within a node, our infrastructure dynamically allocates and re-allocates virtual processors to their physical counterparts. Across nodes, our infrastructure employs live migration to relocate complete guest operating system instances to distinct physical hosts.