PID control is used to keep a value in a certain range (eg speed, position ect...) you would probably want to use the x difference and y difference values separately (difference being between the observed position and desired position) then you can use the output to control the x and y motors respectively, be careful because unless metered the PID can "wind up" if its in an extreme value too long causing it to want to over shoot and possibility "oscillate" more info
here