At the time of writing proxmox is an excellent hypervisor and there are many guides to set it up. You should check that out if you havent. However if you just want a web GUI for kvm and or to monitor usage on linux hosts like network/disk/cpu etc then read the below.
Using cockpit if you have used Virt-Manager (Virtual Machine Manager) is very easy just install it such:
sudo apt-get install cockpit cockpit-machines cockpit-pcp cockpit-system cockpit-storaged cockpit-networkmanager
(For just monitoring a remote linux machine you dont need the cockpit-machines package)
Start it up:
sudo systemctl start cockpit
Access http://127.0.0.1:9090/ to login with the local account you already use. You can also choose 'connect to' to connect to a remote instance. Normally you would start with your main host and then add additional when logged in.
For example when logging in you can click the dropdown in the top left with the name of the account you are logged in as and choose: add new host. You enter any other hosts you want to add so you can switch between them.
The rest should be pretty basic. Assuming you have the VM's stored on a SAN or NAS unit (ie shared storage) you can migrate machines between hosts and see whats running what. Its not as super full featured as enterprise systems yet but works fine and will improve over time. There is a plugin called ovirt which does not yet work for debian afaik which makes it more enterprise but you can use it on RHEL. You may find that cockpit is simple/good enough for you.
Regarding ovirt - its is a full stack so there is not just the web interface - there is the hypervisor, guest additions and other bits and bobs etc. It is possible to run the ovirt-engine as a VM on one of the KVM hosts which could be your only RHEL VM. This would give a more pure debian style KVM host/one vm acting as a controller type setup. An alternative might be Proxmox (which is better imho). If ovirt ever runs on debian this VM would also not be needed.
One nice thing about cockpit is it can provide basic trends of servers over a period of time so you can monitor disk activity etc say over 1 day or 1 week and see trends eg:
This can help with trends of servers over time.