Compared to Xen and KVM, VirtualBox runs much larger parts of the device emulation in ring 0, together with the core CPU virtualization code, for performance.

Compared to Xen and KVM, VirtualBox runs much larger parts of the device emulation in ring 0, together with the core CPU virtualization code, for performance. This is particularly important when KVM is used with a special user (such as user "qemu") and SELinux, because then a bug in QEMU becomes extremely hard to turn into host root access. Libvirt takes care of configuring SELinux this way for you, when you use for example KVM on OpenStack.