Hi and welcome to the Forum
If it's just running virtual machines with KVM, I would not expect issues, aside from having the vm's shutoff while doing the upgrade.

I suggest you first review the release notes and upgrade documentation first;
https://www.suse.com/releasenotes/x8...E-SLES/11-SP4/
https://www.suse.com/documentation/s..._sle11sp3.html

I would also snapshot/backup the vm's, any system configuration changes and important data.