I think the title says it all?
Welcome to the boards. I assume that this question comes from seeing the Free RPG Day product, which has two adventures in it -- one for DCC RPG and the other for the X-Crawl RPG.
The short answer is that what they have in common is that they are both product lines that Goodman Games is promoting. They have nothing in common.
The longer answer is a little more complex. X-Crawl is a d20 based RPG (which means 3E compatible) where characters are sort of like professional showmen. They go on TV on death match competitions and show off their funky powers. Sort of like watching professional wrestling. As a d20-based game, X-Crawl has some roots in common with DCC but not a direct compatibility.
This doesn't mean that you couldn't use the X-Crawl adventure in a DCC campaign, but (1) the flavor is different, and (2) it would require a lot of conversion.
Hopefully that helps?
_________________Marv / FinarvynDCC Minister of Propaganda
; Deputized 6/8/11
DCC RPG playtester 2011, C&C
since 2003, ADRP
Since 1993, OD&D
player since 1975
"The worthy GM never purposely kills players' PCs, He presents opportunities for the rash and unthinking players to do that all on their own."
-- Gary Gygax
"Don't ask me what you need to hit. Just roll the die and I will let you know!"
-- Dave Arneson