Is there a difference between potassium manganate and permanganate?
No, they are somewhat different. $\ce{K2MnO4}$ is potassium manganate and $\ce{KMnO4}$ is potassium permanganate. Notice the two potassium for each $\ce{MnO4}$.

The Roman numerals mark the oxydation state. So, manganate will have a (VI) and permanganate will have a (VII).