That's a distinction without a difference if you're using a modern processor with special virtualization silicon like Intel VT-i/x/d or AMD-V. With these features, IO and memory management is completely isolated between different virtual machines at the hardware level so there is almost zero overhead. With VT-d, which allows you to dedicate DMA channels to a guest VM, you can completely bypass the the host for all types of hardware including PCI/e, USB, etc.