What is the difference between line and phase voltage?

The three 'hot' conductors supplying electricity to a three-phase load are called LINES, not 'phases' (although unfortunately the term 'phase' is widely, but incorrectly, used in the field).

Phases exist between line and neutral in star, or wye, connected systems, and between lines in a delta, or mesh, connected system. Accordingly, the term 'phase to phase' is quite meaningless.

A line voltage, therefore, exists between any two lines.

For a star (wye) connected system, a phase voltage exists between any line and the neutral conductor. For a delta (mesh) system, a phase voltage is exactly the same as a line voltage.