what is the difference between allotrophic form and polymorphic forms

"POLYMORPHISM" means existence of substance into more than one crystalline forms.

That means under different conditions of temperature and pressure, a substance can form more than one type of crystals. This phenomenon is called Polymorphism and different crystalline forms are known as ‘POLYMORPHICS’

Example: Mercuric iodide (HgI₂) forms two types of crystals such as Orthorhombic and Trigonal.

ALLOTROPY means existence of an element into more than one physical forms.
That means under different conditions of temperature and pressure an element can exist in more than one physical forms. This phenomenon is known as Allotropy and different forms are known as "Allotropes".

Example: Coal, coke, Diamond, graphite, lamp black etc. are all allotropic forms of carbon.