See my reference configuration for hardware selections which worked well for me when running Kubuntu 18.04 guests on Kubuntu 18.04 hosts.
After configuring your VM you might like to read about installing Kubuntu on it.
My shared clipboard wasn’t working for my Ubuntu 18.04 guests running on my Ubutnu 18.04 host under VirtualBox 6.0.
To fix first make sure that the shared clipboard is enabled in VirtualBox for the VM. The setting for that is under General -> Advanced -> Shared Clipboard.
Then try running these commands in the guest:
These steps (or something similar) got the shared clipboard working for me. Hooray!
Note that these instructions will vary if you’re running a HWE kernel.