Virt-Manager is simply a GUI front end for KVM that manages virtual machines through libvirt. The GUI might not be the most modern-looking piece of software you'll ever see, but it makes working with ...
My virt-manager created and managed VMs did not show up in Boxes, or vice versa. Apparently you can ask virt-manager to connect to qemu:/// and see the Boxes guest (haven't tried, but have been told); ...