  We suggest new recursive formulas to compute the exact value of the
Kullback-Leibler distance (KLD) between two general Hidden Markov Trees (HMTs).
For homogeneous HMTs with regular topology, such as homogeneous Hidden Markov
Models (HMMs), we obtain a closed-form expression for the KLD when no evidence
is given. We generalize our recursive formulas to the case of HMMs conditioned
on the observable variables. Our proposed formulas are validated through
several numerical examples in which we compare the exact KLD value with Monte
Carlo estimations.
