If you need to use virtual machines on Linux, try Virt-Manager. Virt-Manager is more reliable than VirtualBox. Virt-Manager is completely free and open-source. I'm a long-time supporter of VirtualBox.
KVM is the kernel-based virtual machine, a mainline kernel feature since versuib 2.6.20 for running virtualized guest systems on top of a host system. In less technical terms, this means you can run ...