For a long time I thought there is no such thing as bidirectional clipboard support when using KVM. It turns out I was very, very wrong. All you need to install on Debian-like guest systems is the package spice-vdagent. You may have to start the service afterwards:
apt-get install spice-vdagent
systemctl start spice-vdagentd
Again, this goes on the guest system. On the host system you just have to make sure you’re connecting through spice with a spice client (outdated documentation about that is here). If you use the Virt-Manager GUI, this is all set up correctly by default for Linux guests.
Free bonus: 3D acceleration and compositing seems to be working by default in Linux guests as well. This has nothing to do with spice-vdagentd, I just thought I’d mention it since this seems new. It’s a boon for testing software in multiple desktop environments, especially since many modern desktop environments profit a lot from direct rendering and compositing (or even require it to function properly).