I am using a cloud infrastructure based on XenServer and orchestrated by Cloudstack for lab purposes, and I want to try another KVM hypervisor on the same cloud. Is it possible to use oVirt or Proxmox along with Cloudstack? I mean.. is oVirt/Proxmox similar to XenCenter and able to cooperate with Cloudstack?
CloudStack has native support for KVM, you don't need any additional component such as oVirt or Proxmox. You can deploy/setup new KVM hosts and add them to a new KVM-based cluster in CloudStack. See http://docs.cloudstack.apache.org/en/latest/installguide/hypervisor/kvm.html and http://docs.cloudstack.apache.org/en/latest/adminguide/hosts.html
https://docs.cloudstack.apache.org/en/4.14.0.0/conceptsandterminology/concepts.html#what-can-apache-cloudstack-do says oVirt and Proxmox are both not on the list.
If you are willing to explore other management options, you could try ManageIQ - it supports oVirt, Openstack VMWare, k8s and a bunch of other platforms (No Xen though)