The pixie tool modifies the executable of a program so that every basic block is counted during execution. Its output ranks functions by the absolute count of instructions they executed.
The prof tool samples the instruction counter of the program while the program is executing. Its output ranks functions by the amount of time that the CPU spent in their code.
Normally the output of these tools should agree on the location of the hot spots in a program. However, if prof shows that a function is taking more time than is justified by its pixie execution count, that function may be running slowly due to cache-miss problems.