USB access from guest in Hyper-V

Hyper-V specifically does not allow this. At all. (At least as of Server 2012 R2). The best work-around I've seen is to look for third party software that supports sending usb datagrams over a network connection. There will be an app installed on the host to make the usb device available, and an app installed on the guest that connects to the host and creates a "virtual" usb device in the guest operating system.

I have had luck doing this specifically with a security dongle that a certain software package required be attached. The usb network software we used was from a company called Fabula Tech. Even better, this system can work with any set of machines on the network, so the usb host not need be located on the same machine as the virtual machine guest.

In researching this, I also found small embedded devices with a LAN connection and as many as 10 usb ports, in one case even designed to mount in a rack. The idea was that for shops running systems where a virtual machine may move around to different physical hosts, you weren't tied to keeping your guest on a specific physical box any more just because it needs access to a specific physical usb device, and your physical usb devices weren't tied to machine with a small upgrade cycle and service windows. Then the usb host device itself could be viewed more like an appliance or as part of the cable plant.