[USB Isolation] USB virt drivers access between guests instead of host -> guest?

[privacymiscoccasion@xxxxxxx]

Hi everyone,

I'm coming over from reading about Qubes OS, which uses the Xen 
hypervisor. In Qubes, the way that untrusted devices like USBs are 
handled is that they are pass through to a VM, which then (I presume) 
allows other guests to access them using virtual drivers.

I'm looking for a theoretical explanation on how this would be possible 
with KVM. I am not a developer and thus am having difficulty 
understanding how one would let a guest access virtual drivers 
connecting to hardware devices like USB and PCIe from another guest.

Any help/practical examples of this would be greatly appreciated. This 
seems to be a hard topic to find and so far I haven't come across 
anything like this.

Thanks!