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

This error is caused by having kernel module kvm and VirtualBox at the same time. KVM is a full virtualization solution for Linux on x86 hardware containing virtualization extensions (Intel VT or AMD-V).
http://www.linux-kvm.org/page/Main_Page

To remove kvm module enter:

sudo modprobe -r kvm-intel
sudo modprobe -r kvm-amd

remove kvm from modules that are loaded at boot time. These are stored in file:

/etc/modules