This paper presents Paver, an interactive tool for designing structured patterns composed of 3D elements on free-form 3D surfaces. To address the inherent complexity of direct pattern creation in 3D space, we introduce a parametric design model that allows users to define patterns on a 2D parameter plane. Each pattern is defined by an underlying tessellation and isometric elements. A bijective mapping then lifts these 2D designs onto the target input surface. To ensure structural coherence and accommodate surface curvature, we model the tessellation as a 2D mass-spring system and propose an optimization method to adjust the particle positions on the 2D parameter plane under well-designed system forces. Once optimized, the tessellation is transferred to the 3D surface, generating the final element-based pattern. Paver supports diverse pattern types and offers real-time feedback, enabling designers to iteratively explore and refine complex surface decorations with ease. Experimental results are presented to demonstrate the effectiveness of our Paver.