Pro Res has been built from the ground up to be a CPU decoding and encoding codec. Unlike the MPEG 4 codec's, Pro Res is a video-only codec like DNX and JPEG2000. h264 is an open source variant of the same MPEG 4 codec, only in most cases companies only use the lower end version of it Long GOP 8 bit 4:2:0. It was developed specifically for camera originals due to the efficiency and quality of the codec.

XAVC is part of the MPEG 4 family which includes. Like Pro Res, it has many variations based on bandwidth, progressive segmented frame or i-Frame 8 bit or 10 bit and of course frame size up to 4k. XAVC is a proprietary codec developed by Sony for their cameras. if you shoot the more compressed ones like the 100Mbps ones then there is lots more difference this is for the 300-400Mbps 4k/UHD xavc varieties. workability in post is pretty much the same in most situations I think though you may find differences if the material is not optimally shot. So I would put it pretty close to the Prores422 on that scale if it is properly exposed. Nowadays I shoot a lot with an external recorder on Prores422hq and it is slightly better quality than xavc especially if underexposed though will take significantly more storage spage.

I haven't pixel peeped them side by side with material shot in the same situation (would require the same camera to record both at the same time which has not been practical yet in projects I do). The better xavc varieties may be visually very similar looking in quality compared to the prores422 unless you underexpose the xavc camera a lot which may generate weird compression artifacts like blue dots flashing around.

Xavc s vs avchd vs mp4