Run AVD and VirtualBox at the same time

You can not have two kernel modules try to use the vt-x extension at once. So you can't have two hardware assisted virtual machines ran by two different hypervisors at the same time, on the same node.

However, you can remove the module that is currently using the extension via modprobe(and friends). This would free up the extension for use by the other hypervisor.

For kvm the modules you would need to remove are: kvm, kvm_intel or kvm_amd. then virtualbox would be allowed to use vt-x.

The same goes for whatever modules virtualbox inserts into the kernel.

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You can use qemu(the userland tool commonly paired with kvm) with software emulation. it does that by default when you don't specify -machine accel=kvm or -enable-kvm and you will have a virtual machine without hardware assisted acceleration.

I am not very familiar with virtualbox so i am not sure of your options with it, for non-accelerated virtual machines.