4.7.4. USB Device Passthrough
The USB device passthrough capability allows a physical USB device from the host machine to be assigned directly to a guest machine. The guest OS drivers can use the device hardware directly without relying on any driver capabilities from the host OS.
USB devices are only inherited by the guest domain at boot time. newly activated USB devices can not be inherited from the host after the guest domain has booted.
Some caveats apply when using USB device passthrough. When a USB device is directly assigned to a guest, migration will not be possible, without first hot-unplugging the device from the guest. In addition libvirt does not guarantee that direct device assignment is secure, leaving security policy decisions to the underlying virtualization technology.
