Which algorithm Doc2Vec uses?
Word2Vec is not a combination of two models, rather both are variants of word2vec. Similarly doc2vec has Distributed Memory(DM) model and Distributed Bag of words (DBOW) model. Based on the context words and the target word, these variants arised.

Note: the name of the model maybe confusing 


Distriubted Bag of words is similar to Skip-gram model
Distributed Memory is similar to Continuous bag of words model