Kali Linux is a wonderful choice to be used within a virtual machine. While most Windows users might prefer to run it in VMware or VirtualBox (or Hypervisor), Linux users have the choice to use QEMU/KVM.
What to install
I often had problems installing the guest additions for VirtualBox. In QEMU/KVM those additions are called spice-vdagent. Once installed, the virtual machine can adapt to the host’s window size. Other features are copy and paste between host and guest as well as file transfer. More features can be found here:
Installing vdagent is very simple in Kali Linux. After installation first update the system and reboot. Then open a command line and type:
apt-get install spice-vdagent
Reboot to make the new features available.