Chapter 7. Overcommitting with KVM
The KVM hypervisor automatically overcommits CPUs and memory. This means that more virtualized CPUs and memory can be allocated to virtual machines than there are physical resources on the system. This is possible because most processes do not access 100% of their allocated resources all the time.
As a result, under-utilized virtualized servers or desktops can run on fewer hosts, which saves a number of system resources, with the net effect of less power, cooling, and investment in server hardware.