What standard SQL isolation levels do the SQLite isolation levels correspond to?
DEFERRED, IMMEDIATE, and EXCLUSIVE are not isolation levels. All three modes ensure the serializable isolation level. (SQLite's transcations are always serializable because it actually serializes them; there is a single, database-wide lock.)

IMMEDIATE takes the equivalent of a write lock at the beginning of the transaction; this prevents a deadlock when two concurrent read-only transactions try to update to a write lock. EXCLUSIVE is useful when you want to lock the database without actually writing.