So ... you gave the guest 3.7G on a 4G system? I would recommend against that. Always leave the hostOS with 1G of RAM and 1CPU if you have more than 2 cores.
Read this and this. The main ideas for the virtualbox article apply to KVM ... or any virtualization, just not the exact details.
No need for 50G of storage, just to try out Ubuntu. 10-15G is fine. Here is my main desktop machine:
I've been using this since 2008. For a long time it was 10G.
$ df -h
Filesystem Size Used Avail Use% Mounted on
/dev/vda1 14G 11G 1.9G 86% /
BTW, I run it inside KVM in the way that the first link explains.
The main ideas are:
* Preallocate all storage. There are exceptions in the article for SSD, otherwise, preallocate.
* Use virtio drivers for Linux storage and network drivers
* Do not hog the hardware. allocate 1 vCPU and 1G of RAM to a guestOS unless you KNOW more is needed.
* Use virtual ICH chipsets when possible, not older models.
* Virtual hardware matters, forget any real hardware. The VM does NOT care.