Monitor the resource usage by VMs in KVM

this is not your final answer - I would start by looking at disk iops - use tools like iostat and sar (sysstat) - find out what your practical iops are - and check if your swap is busy also

I have seen similar behavior with sata drives in a software raid (md), every time a kvm guest was doing a lot of writes the vm's responsiveness lagged quite a bit. I didnt take the time to tweak, we switched to a raid controller with cache, battery and writeback - still the same drives, that helped a lot - my hunch is the caching of writes was the trick. (that was a single quad xeon with 12GB of ram running 2 windows DC's and 5 small application servers, none was all that busy, used a 3ware 9690).

KVM is a hypervisor, not a management stack, so if you want to manage it you either log in to the host console, or use management tools like libvirt. Comparing it to ESXi is not very correct in this sense - ESXi is packed with daemons and additional tools, and this is exactly the functionality libvirt provides (actually, it provides more), so I can't see why you cannot use a common Linux based library that is designed to answer your every need on a single host setup.

KVM has the basic command line tools to start/stop VMs, and to interact with the VM you can connect to qemu monitor, however, all of this is much easier and intuitive with libvirt as well.

BTW, using libguestfs you can read the guest filesystem from the host, and take selective backups. And with libvirt managing VM snapshots and lifecycle is also much easier than without it.