Is a woman considered a virgin or a non-virgin if a lawful seclusion and partial penetration of the penis occurred without breaking the hymen before divorce?

A woman is not considered a "thayyib" (previously married) unless she has lost her virginity through marriage. This is the difference between a "bikr" (virgin) and a "thayyib": a bikr has not lost her virginity through marriage, whereas a thayyib is one whose virginity has been lost due to it.