What are the existing types of transaction isolation levels in database management systems?

In database management systems (DBMS), there are four main levels of transaction isolation, which determine how and when changes made by one transaction become visible to other transactions:

1. Read Uncommitted — the lowest level of isolation. A transaction can see changes made by other transactions even if they are not yet committed (dirty reads).

2. Read Committed — a transaction only sees changes that have been committed. This prevents dirty reads but allows non-repeatable reads.

3. Repeatable Read — guarantees that data read in a transaction will not change until the transaction completes. It prevents non-repeatable reads but allows phantom reads.

4. Serializable — the highest level of isolation. Transactions are executed as if they were sequential, one after another. It prevents all types of anomalies but reduces concurrency and system performance.

The choice of isolation level depends on the requirements for data consistency and system performance.