In your previous thread, I explained what a NOVALIDATE constratriant is "A NOVALIDATE constraint is basically a constraint that can be enabled but for which hive will not check the existing data to determine whether there might be data that currently violate the constraint"
The difference between a UNIQUE constraint and a Primary Key is that per table you may only have one Primary Key but you may define more than one UNIQUE constraint. Primary Key constraints are not nullable.UNIQUE constraints may be nullable. Oracle also implements the NOVALIDATE constraint here is a write-up by Richard Foote
When you create a UNIQUE constraint, the database automatically creates a UNIQUE index. For RDBMS databases, a PRIMARY KEY will generate a unique CLUSTERED INDEX. A UNIQUE constraint will generate a unique NON-CLUSTERED INDEX.