Runing Virtualbox virtual machines in Windowless mode AKA headless mode has always a much needed option. You could do it from a command line in Linux. Starting from version 4.2.0, VirtualBox has added support for starting of headless VMs from the GUI. Running a virtual machine in headless mode is a better way to save and reduce consumption of resources on physical computer. To start a virtual machine in headless mode, select the VM you want to run and just simply press and hold SHIFT key, then press start.
You can shutdown a VM which is running in headless mode by ‘ACPI Shutdown’ or ‘Power Off’ options. The ACPI Shutdown method is the best method to shutdown, which allows the guest VM to close down gracefully.