Unless you can find an old version of Windows NT for PowerPC, the POWER8 won't run any commercially released version of Windows.
Not even then. The PowerPC port of NT was designed for CHRP;
Splitting hairs here, but Windows NT for PowerPC was actually designed for PReP. It was ported by Motorola and not Microsoft. Due to the lack of actual PReP hardware (ie Apple backed down from releasing Mac OS), it was dead in the market with just Windows NT being the sole major platform to get any traction on it.
I don't believe that PowerVM provides a CHRP-like base (I don't know for certain, mind, and I'm happy to be corrected if it does, but it would greatly surprise me.) It takes more than just support for the ISA to make a workable operating system - that's a necessary, but not sufficient, step.
Apple said that they couldn't support PReP even though they were originally on board. CHRP was an extension of PReP to further support more MacOS specific features. One of the main changes was that Open Firmware was to be required under CHRP. It was optional under PReP (and for reference Windows NT on PReP did use OpenFirmware to boot
CHRP has been used by IBM for the POWER4 and POWER5 systems. IBM further extended CHRP with PAPR in 2006 which was used on the JS20/21 PowerPC 970 blades, POWER6 and later servers. Source: AIX 7.1 release notes.
and its references of CHRP on modern POWER hardware.
The firmware and hardware is there to run Windows NT under PowerVM. The limitation at this point I see is if PowerVM can pass itself as a 32 bit platform as Windows NT 3.5/4 obviously has no 64 bit support. I believe that it can due to its ability to boot older 32 bit versions of AIX for backwards compatibility reasons. Little endian support is also needed for Windows NT. I just have no experience with PowerVM to know first hand.
(For the curious: I've done a little AIX admin work, and am certified - at a basic level - with AIX 6.1. Never done a huge amount with it, though; I know Solaris better, and even that was getting on over five years ago.)
Having worked at a remote hosting facility, it got to work with a wide abundance of hardware/software myself. I got to deal with pretty much everything under the sun there in terms of Linux and Windows as a jack of all trades technician. The rare stuff I've dealt with Solaris on SPARC + x86, and some AIX on POWER6 + POWER7 hardware there. Before that I worked in the IT side of the insurance industry where I learned the bare minimum about OS400 and zOS for some daily activities.
I have yet to use an Itanium system and neither OpenVMS or HPUX on any particular set of hardware.