Windows PV Drivers 8.1.0
This is the first release in the 8.1 series.
To install a driver on your target system, unpack the tarball, then navigate to either the x86 or x64 subdirectory (whichever is appropriate), and execute the copy of dpinst.exe you find there with Administrator privilege. More information can be found here.
You can also get the drivers via https://xenbits.xen.org/pvdrivers/win/8.1.0.