One of Hardy spaces H_2 is a Hilbert space (the norm can be generated by an inner product: ||f|| = \sqrt ) For the the remaining values of p, the corresponding H_p is a Banach space, but is not a Hilbert space. The standard reference to the general theory of Hardy spaces that I know is the following book:
Koosis, P. (1999). Introduction to Hp Spaces (2nd ed., Cambridge Tracts in Mathematics). Cambridge: Cambridge University Press. doi:10.1017/CBO9780511470950
In complex analysis, the Hardy spaces Hᵖ are certain spaces of holomorphic functions on the unit disk or upper half plane where as A Hilbert space is a vector space equipped with an inner product, an operation that allows defining lengths and angles.
In mathematics, a Hilbert space is an inner product space that is complete with respect to the norm defined by the inner product. Hilbert spaces serve to clarify and generalize the concept of Fourier expansion and certain linear transformations such as the Fourier transform. https://www.quantiki.org/wiki/hilbert-spaces
In complex analysis, the Hardy spaces (or Hardy classes) Hp are certain spaces of holomorphic functions on the unit disk or upper half plane. They were introduced by Frigyes Riesz, who named them after G. H. Hardy, because of the paper. In real analysis Hardy spaces are certain spaces of distributions on the real line, which are (in the sense of distributions) boundary values of the holomorphic functions of the complex Hardy spaces, and are related to the Lp spaces of functional analysis. For 1 ≤ p ≤ ∞ these real Hardy spaces Hp are certain subsets of Lp, while for p < 1 the Lp spaces have some undesirable properties, and the Hardy spaces are much better behaved. https://en.m.wikipedia.org/wiki/Hardy_space
D(α) is the weighted Dirichlet space defined by the weight (1 − |z|2)α. D = D(0) is the classical Dirichlet space and D(1) = H2(D) is the Hardy space. Reproducing kernel and reproducing formula. ... Theorem 16 H is a reproducing kernel Hilbert space iff it has bounded point evaluation at all z ∈ Ω. http://www.dm.unibo.it/~arcozzi/hilbert.pdf