Thanks Peng Xu for teaching me about these three models. The differences between them are as follows:
- HMM: arrow from st-1 to st, and from st to ot
- Maximum Entropy HMM: arrow from st-1 to st, but from ot to st
- CRF: no arrow between st-1 and st, neither between st and ot
- Andrew Mccallum, Dayne Freitag, Fernando Pereira. Maximum entropy Markov models for information extraction and segmentation, ICML 2000.