Guess what, next to impossible.
The way the modern/metro touch keyboard is implemented does not allow applications to explicitly show it. It's true, I'm not making this up:
Additionally, when you're in full-screen mode, overlaying something like an on-screen keyboard from the host will almost certainly cause problems:
Many desktop games will be written using full-screen exclusive mode. This presents a problem when using the built-in keyboards, because either the invoked keyboard will be hidden, or raising the keyboard will force the game out of full-screen mode. https://software.intel.com/en-us/blogs/2013/06/11/touch-keyboard-access-for-windows-8-desktop-apps
I know the above article talks about games but the concept is the same, exclusive full-screen desktop apps don't like sharing the display (or more accurately, they can't).
Anyway, in the end, you might as well just use the On-Screen Keyboard provided within the Windows XP guest itself: