。
My opinion is completely different from yours.
Literal
I [of-の] opinion [topic-は] your one [from-とは] completely differs.
君の uses noun ellipsis (君の = 君の意見) — the second 意見 drops because it's recoverable from the first. The とは here is と ('with/from' — standard of comparison) plus the contrastive は, marking 'from yours specifically, not someone else's.' 全く違う is the standard emphatic phrasing for total difference.