Well, we are at the end. As said before, this article is a sort of experiment, as the testing methodology can surely be improved. If you have any idea, let me know it!
After having seen the results, we can say that you had to stay away from BTRFS in your KVM virtualizer host: this is not unexpected, as the KVM tuning page warns about very low BTRFS performance in these scenarios.
On the other hand, both EXT3/4 and XFS are more or less equivalent choices. The EXT-based filesystems have very good performance in both random and sequential tests, while XFS has an edge in the random write test and in fragmentation level.
Anyway, please remember that different kernel version can led to very different results, so things can always change in the future.
Have a nice day!