While attempting to run up KVM on a CentOS install tonight I found that Linux was reporting the processor did not offer hardware virtualization support;

[root@srv ~]# egrep '(vmx|svm)' --color=always /proc/cpuinfo
[root@srv ~]#

This is despite running a processor which has full support per the vendor, as well as it being enabled within BIOS. It is important to note that when running a XEN kernel, support for the extensions will not be reported. A quick reboot into a non XEN kernel and you will be back in business.