Optimizing a compare query
Not sure how such a query should be optimized for Oracle, when no indexes are available but here's another rewriting. It will not calculate the number of mismatching tuples but will stop searching when it finds one:

SELECT 1
FROM dual
WHERE EXISTS
     (
        Select
            X, Y, Z
        From
            A
     MINUS
        Select
            X, Y, Z
        From
            B
     ) ;


SQL-Fiddle shows a similar performance for all the provided answers, as the two tables have to be fully scanned in the worst case.

Rolando's and Narendra's queries show different execution time (sometimes better, sometimes worse), I think because of the HASH JOIN used.  

There may be different results with bigger tables and with fewer or more matches between the two tables. (the test were done with only a few K rows).