During code review tasks, comparing two versions of a hardware design description using existing program differencing tools such as diff is inherently limited because existing program differencing tools implicitly assume sequential execution semantics, while hardware description languages are designed to model concurrent computation. We designed a position-independent differencing algorithm to robustly handle language constructs whose relative orderings do not matter. This paper presents Vdiff, an instantiation of this position-independent differencing algorithm for Verilog HDL. To help programmers reason about the differences at a high-level, Vdiff outputs syntactic differences in terms of Verilog-specific change types. We evaluated Vdiff on two open source hardware design projects. The evaluation result shows that Vdiff is very accurate, with overall 96.8% precision and 97.3% recall when using manually classified differences as a basis of comparison.