Yes, 0 is not positive, but does that explain the result of PowerMod[0, 0, 7]?
The reason is that 0 and 7 are congruent modulo 7, so in the context of the ring Z/7Z, 0^0 can be thought of as the same as 7^0. Thus we obtain the same result as PowerMod[7, 0, 7].
With Mod the situation is different, since 0^0 evaluates to Indeterminate first, so we are asking for Mod[Indeterminate, 7].
