Hardness is generally the resistance of the material to penetration by a test body. The advantage of the Vickers hardness test is that the entire hardness range encountered in the screw is covered by the method. For details see ISO 898, part 1.
Vickers hardness HV: ISO 6507
(encompasses the complete hardness range usual for screws)
Brinell hardness HB: ISO 6506
Test body ball
Rockwell hardness HRC: ISO 6508
Test body cone