Tiggerito - 8:37 pm on Aug 6, 2012 (gmt 0)
You're not using the technical terms so we have to guess exactly how this is done.
I'm suspecting xyz.com just contains a frame or iframe that wraps abc.com.
If this is the case, most search engines will know this and consider it as nothing new. It will get ignored.
So, it won't help in your ranking but should not cause any issues with it either.
You should also consider usability though. Wrapping websites in frames cause problems like making it impossible to bookmark pages.