There is no difference in the end ; both forms amount to describing the same context ; however it is done according to two ways of looking at the context.
- Il en existe des/de grands. Here, the article is necessary; it's a partitive.
- Il en existe des/de petits.
If we consider the two example together, we can say we have two types, the type of the big ones and the type of the small ones. If we refer to the types only, we say « Il en existe deux types. ». We can consider however the elements themselves as belonging to two categories and for that a préposition is needed ; it has the same form as the partitive article "de", but it is not the same word ; we then say « Il en existe de deux types. »
It is similar in English : They exist in two types (They come in two types.). "They exist" corresponds to "Il en existe"; only the preposition is different.
(TLFi) b) De marque la descendance, la filiation; l'origine. Jésus-Christ qui pour le salut de tous les hommes est né de la très Sainte Vierge Marie