AMD-V and Intel VT virtualisation extensions

http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5.4/html/Virtualization_Guide

AMD-V or Intel VT virtualisation extensions are required for full KVM or Xen virtualisation.

Use $ cat /proc/cpuinfo to see what your CPU supports. You need either 'svm' or 'vmx' but note that some older Intel XEONs do not have this !