The extended finite element method (XFEM) models crack propagation in a finite-element mesh without domain re-meshing. The discontinuous and singular elastic fields, associated with cracks, are represented by using an enriched shape function space that includes discontinuous and singular functions. This talk presents a fully coupled 2D XFEM model of hydraulic fracture propagation capable of simulating propagation in various regimes including toughness- and viscosity- dominated propagation. In order to accurately capture the singular pressure at the fracture front, such as that in the viscosity-dominated regime, a mixed coupled XFEM scheme is developed. The required singular enrichment shape functions are derived that describe power-law singularities in the displacement and stress fields near the fracture front. An alternative enrichment strategy is investigated for efficiency, in which the crack is represented by only a discontinuous (sign) enrichment and the exact location of the fracture front is prescribed in the sign-enriched XFEM in a weak sense.