VirtualBox can't operate in VMX root mode

OS:-RHEL5.8-64bits
OVM VirtualBOX:- 4.3.26

When starting a virtual machine, VirtualBox will complain:

"Failed to start the virtual machine XYZ

VirtualBox can't operate in VMX root mode. Please disable the KVM kernel extension, recompile your kernel and reboot.
VBox status code: -4011 (VERR_VMX_IN_VMX_ROOT_MODE).

SOLUTION:-

The error message means that there is another VM host software that occupies the CPU's resource, which can be used for CPU-level virtualization. You can check if there are any other VM software on your system, for example, if you install more than one VM software on the same machine (VirtualBox, VMware, Parallels etc.), then only one of them can take CPU's resource to virtualize Guest OS. Uninstalling all the other VM software may solve the issue.

This error is because two virtualization technologies are present on your system (KVM and VirtualBox).

If you need both installed, you can disable KVM temporarily by running:
$ sudo /etc/init.d/qemu-kvm stop

If you don't really need KVM, you can uninstall it by running:
$ sudo apt-get purge qemu-kvm

It's the same. "/etc/init.d/qemu-kvm stop" calls "stop qemu-kvm".

Try disabling kvm module...
"modprobe -r kvm_intel" with root privileges.