I don't know about kivy specifically, but we're currently working on yuv support through the gl driver and kms. Predominantly this means support for dmabufs in order to avoid multiple image copies around the place.
You will need to rebuild Mesa to get the latest goodness, but https://github.com/anholt/drm_mmal/tree/x11?files=1
should work as a simple demo of getting video decode into a window via gl. Use ninja to build it, not the makefile (look at the git history for the full runes required).
Now mmal is Pi specific. I'm 95% of the way to having a v4l2 codec driver sorted, so it'd be worth checking of kivy can handle that directly.
Software Engineer at Raspberry Pi Trading. Views expressed are still personal views.
Please don't send PMs asking for support - use the forum.
I'm not interested in doing contracts for bespoke functionality - please don't ask.