Question

whats the difference between biconvex lens and double convex lens

Answer 1

There is plano convex, and bi- convex. 
The term "convex" is commonly used (even if incorrectly), to denote a bi- convex lens. Any other arrangement needs to be more explicit, e.g. plano- concave, plano- convex, bi- concave, concavo- convex, etc.

Answer 2

'nothing' both are same .. names r different...