Hybrid AC/DC distribution networks (HDNs) have emerged as a promising architecture for integrating distributed energy resources and the growing number of DC loads. However, most existing expansion planning studies treat demand response (DR) as a homogeneous flexibility resource, neglecting the different flexibility characteristics of AC and DC loads. This simplification may lead to unrealistic estimations of demand-side flexibility and suboptimal planning decisions. This paper proposes an expansion planning framework for HDNs that incorporates differentiated DR modelling for AC and DC loads. In the proposed approach, distinct flexibility limits and participation costs are assigned to AC and DC loads to better represent their controllability characteristics. The planning problem is formulated as a mixed-integer nonlinear optimization model that determines network expansion decisions by incorporating DR utilization. The model is solved using a genetic algorithm implemented in MATLAB, while operational costs are evaluated through an optimal power flow module developed in GAMS. Three planning scenarios are analyzed to assess the effectiveness of the proposed framework: without DR, with uniform DR, and with differentiated DR for AC and DC loads. The results show that modelling differentiated DR improves demand-side participation and reduces the net present value of total planning costs compared with conventional uniform DR modelling. These findings highlight the importance of accurately representing heterogeneous demand flexibility in the planning of HDNs.