JVM is the cornerstone of the widely-used Java platform. Thus, it is critical to ensure the reliability and robustness of popular JVM implementations. However, little research exists on validating production JVMs. One notable effort is classfuzz, which mutates Java bytecode syntactically to stress-test different JVMs. It is shown that classfuzz mainly produces illegal bytecode files and uncovers defects in JVMs’ startup processes. It remains a significant challenge to effectively test JVMs’ bytecode verifiers and execution engines to expose deeper bugs. This paper tackles this challenge by introducing classming, a novel, effective approach to performing deep, differential JVM testing. The key of classming is a technique, live bytecode mutation, to generate, from a seed bytecode file f , likely valid, executable (live) bytecode files: (1) capture the seed f ’s live bytecode; (2) repeatedly manipulate the control- and data-flow in f ’s live bytecode to generate semantically different variants; and (3) selectively accept the generated variants to steer the mutation process toward live, diverse variants. The generated variants are then employed to differentially test JVMs. We have evaluated classming on mainstream JVM implementations, including OpenJDK’s HotSpot and IBM’s J9, by mutating the DaCapo benchmarks. Our results show that classming is very effective in uncovering deep JVM differences. More than 1,800 of the generated classes exposed JVM differences, and more than 30 triggered JVM crashes. We analyzed and reported the JVM runtime differences and crashes, of which 14 have already been confirmed/fixed, including a highly critical security vulnerability (CVE-2017-1376).