What is the difference between a lemon pound cake, a lemon loaf, and a lemon bread?
The difference is the person doing the naming. Seriously, you seem to be looking for clear cut categories where none exist. 

If one of the same person calls three different things a "lemon bread", "lemon loaf" and "lemon cake", you can typically expect that the "bread" will be somewhat less sweeter, less aromatic and less fine than the "pound cake". This is probably connected to soda breads eaten in times of shortages (especially wars) which were then mixed with vegetables or fruit, but were not as fine as cakes. The "loaf" part may be more connected to the shape than to the batter type. But if two different people are doing the naming, all bets are off. So if you are looking for recipes, you should use all three search terms. 

See also my recent answer explaining a bit more about naming.