A short answer would be that Hermeneutic phenomenology is more concerned with the meaning of the experience of a particular phenomenon, and Existential phenomenology is more concerned with the lived experience of being in the world
I agree with Stephen David Edwards, and might I expand on his 'short answer'.
For the ancient Greeks, Hermes was the Olympian deity able to communicate the wisdom of the Gods to the world below. It is from that perspective, hermeneutic, that the 'lived experience' of existential phenomenology is interpreted as the manifestation of 'Being' (in the eternal sense) in the world.
As far as I know, in the pages of M. Heidegger`s work: Being and Time (paragraph 7), the phenomenological description becomes an interpretation of a distinguished entity: Dasein, initiating contemporary philosophical hermeneutics.
I think there is a difference of subject and goal. Hermeneutic phenomenology is still aiming to uncover some lost objective knowledge through the analysis of subjective experience. Therefore its subject of analysis, although analysed from a contextualised position, is still supposedly universal. For instance being, time, meaning,... Existential phenomonelogy, however, is aimed more at articulating a particular point of view of 'an existence.' So rather than constituting a more traditional epistemological project of universality and objectivity on the ground of subjectivity (like the hermeneutical project), it aims to uncover inherently subjective experiences of existence.
I agree with Daan Van Cauwenberge, and would like to add that as 'inherently subjective experiences of existence' necessarily admit differentiation, the experience and interpretations of the same phenomena will also differ, and therefore ought to be understood as aspects only of that particular body of knowledge.
The question can be rephrased thus: What is the difference between Heidegger and Gadamer? The former centers on time and death; the latter on dialectics and communication. Do not trust my answer. I am a naturalist philosopher and have read just enough Heidegger and Gadamer to bore a team of scientists: that is, just a few pages.