Dedoimedo posted a tutorial explaining how to use Kernel-based Virtual Machine (KVM) and VirtualBox virtualization products side by side in the same session

KVM and VirtualBox supposedly cannot coexist, or rather, VirtualBox won't run while the KVM modules are loaded in memory. KVM can work fine alongside VirtualBox, so it's a one-way problem, really. Solving it is easy too. Unfortunately, I've come across many terrible tutorials that recommend uninstalling either this or that product. And among the saner guides that go for a less stringent solution, most simply tell you rmmod this or that, but they don't tell you how to get things back running again.

I will show you how to use KVM and VirtualBox side-by-side, without uninstalling any one or rebooting in between uses. You will learn how to disable and enable the products so the conflicting counterpart can run. In vivo, no reboots. Let's do this.

  Using KVM and VirtualBox side by side