Historical materialism is historicity without history. And yet it is by no means genealogy. Of course, isnofar as it is historicity without history — which is the historicity of politics as permanent excess — historical materialism as a historiographical textuality is bound to have a genealogical form. But thought in terms of its practical actuality, the historicity of genealogy is merely a facticity or phenomenality of interiorised experience of difference. Precisely for this reason, genealogy is no more than the obverse of historicism. On the other hand, the historicity of historical materialism is now-time as the uninterrupted ongoingness of determinate excess of presence in its historical index — i.e. infinite difference and infinite deployment of infinite difference in its historical index.

The two citations below — from Adorno and Benjamin respectively — enable one to think this radical distinction between post-Hegelian phenomenology of Heidegger, and historical materialism, and thus, by extension, between genealogy (a phenomenologically reductive hermeneutic of difference) and historical materialism:

“…[Benjamin] seems to converge with the general intellectual current which protested against idealism and epistemology, demanding ‘the things themselves’ instead of their conceptual form, and which found an academically respectable expression in phenomenology and the ontological schools stemming from it. But the decisive differences between philosophers have always consisted in nuances; what is most bitterly irreconcilable is that which is similar but which thrives on different centres; and Benjamin’s relation to today’s accepted ideologies of the ‘concrete’ is no different. He saw through them as the mere mask of conceptual thinking at its wits end, just as he also rejected the existential-ontological concept of history as the mere distillate left after the substance of the historical dialectic had been boiled away.”
–Adorno, ‘A Portrait of Walter Benjamin’ (Prisms)

“What distinguishes images from the “essences” of phenomenology is their historical index. (Heidegger seeks in vain to rescue history for phenomenology abstractly through “historicity”.)….”
— Benjamin, Convolutes N, ‘On the Theory of Knowledge, Theory of Progress’ (The Arcades Project)