On my own experience, Kubuntu 13.04 Alpha 1 can be installed through Virtualbox easily. There was no problem when I tried to install this distro on Virtualbox 4.2.2. As usual, after installing a new distro on my Virtualbox, the next main thing is to install the Guest Additions on the guest system. It's the most important thing since we can get better performance and visualisation after installing Guest Additions. OK here is how to install Virtualbox Guest Additions on Kubuntu 13.04 Alpha 1.
1. Boot the Kubuntu 13.04 Alpha 1 guest machine.
2. Go to Devices | Install Guest Additions on the Virtualbox window
3. If the autorun prompt does not work, you can open or browse the content of the Virtualbox Guest Additions on Kubuntu 13.04.
4. Double click the autorun.sh and type the password for root user. The installation will start shortly. But, you may encounter error that looks like this: gcc is not found etc. You can fix this error by installing the following packages.
sudo apt-get install gcc build-essential
5. Now re execute autorun.sh and you should be fine after that.