JOIN 2 tables, and query data from them 3 times
Given your table structure, this should extract what you are looking for:

SELECT DISTINCT p1.value
FROM postmeta p1
INNER JOIN postmeta p2 ON p1.post_id = p2.post_id
WHERE p2.key = 'make'
    AND p2.value = 'Chevy'
    AND p1.key = 'model';