You might try piping stdout AND stderr to the bit bucket like this ...

xcopy ... >nul 2>nul

[by the way, the correct syntax is >NUL not >NULL. If you look, you'll probably find a file call NULL in your default directory somewhere]. I make this mistake all the time

If this doesn't help, post your code or at least some of your xcopy lines for all to see.

Shawn.