SmartOS incorporates the four most revolutionary OS technologies of the past decade — Zones, ZFS, DTrace and KVM — into a single operating system, providing an arbitrarily observable, highly multi-tenant environment built on a reliable, enterprise-grade storage stack. With the introduction of KVM in SmartOS, you no longer have to give up the power of an enterprise-grade operating system in order to run legacy applications and stacks.

via SmartOS.org.