LXC is fine, no need to use KVM if he do not want to.
Your issue above show the mariadb repository, which isnt available, not the hestiacp one. I just checked both and they are up and running fine. So it sounds like a network connection issue on your side. Can you reach the repos manualy if you try to visit them in your browser and/or checking from cli with curl/lynx? Or are you running there in a timeout aswell?
I use it for agility and for the ease in handling data … also if it is for your own use or for a single user … if it is an installation for multiple users or to share resources I recommend KVM
KVM is a hypervisor, Hestia is a hosting control panel, so I don’t see any reasone to prefer one over the other, they work excelent together or were you referring to LXC and not Hestia?
As a hypervisor I use Proxmox or ESXI (visual interfaces easy to manage the KVM virtualization engine and the LXC / LXD container system) … they are similar to the panel that you have developed HESTIACP (an interface to handle programs like nginx, apache, php, sql … etc) … or at least it’s the same concept
I recommend Proxmox, it’s much better than being blind in the KVM terminal… try it, you will not regret it