As server consolidation using virtual machines (VMs) is carried out, software aging of virtual machine monitors (VMMs) is becoming critical. Performance degradation or crash failure of a VMM affects all VMs on it. To counteract such software aging, a proactive technique called software rejuvenation has been proposed. A typical example of rejuvenation is to reboot a VMM. However, simply rebooting a VMM is undesirable because that needs rebooting operating systems on all VMs. In this paper, we propose a new technique for fast rejuvenation of VMMs called the warm-VM reboot. The warm-VM reboot enables efficiently rebooting only a VMM by suspending and resuming VMs without accessing the memory images. To achieve this, we have developed two mechanisms: on-memory suspend/resume of VMs and quick reload of VMMs. The warm- VM reboot reduces the downtime and prevents the performance degradation due to cache misses after the reboot.