1. Key difference 1: Language compatibility FindBugs is primarily designed for Java code analysis, while PMD supports multiple programming languages such as Java, JavaScript, XML, SQL, and others. This means that PMD can be used for code analysis in a wider range of projects that use different programming languages.

2. Key difference 2: Rule sets FindBugs provides its own set of pre-defined rules for identifying common coding issues and bugs in Java code. On the other hand, PMD also offers a set of pre-defined rules, but it allows users to define their own custom rules as well. This flexibility of creating custom rule sets sets PMD apart from FindBugs.

3. Key difference 3: Analysis approach FindBugs analyzes compiled bytecode and detects issues by examining the bytecode instructions. It performs static analysis without the need for executing the code. PMD, on the other hand, analyzes the source code directly and performs rule-based analysis on the code structure and patterns. This difference in analysis approach can result in variation in the types of issues detected by each tool.

4. Key difference 4: IDE integration FindBugs provides direct integration with popular Java development environments, such as Eclipse, IntelliJ IDEA, and NetBeans. It allows developers to run the analysis from within the IDE and view the results seamlessly. PMD also offers IDE integration but supports a broader range of programming languages beyond Java. Therefore, PMD's IDE integration may not be as comprehensive as FindBugs for Java-specific projects.

5. Key difference 5: Performance FindBugs is known for its fast analysis and low overhead on resources. It is capable of analyzing large codebases efficiently. PMD, on the other hand, can be slower compared to FindBugs due to its more extensive rule-based analysis on the source code. The performance difference may vary depending on the size and complexity of the codebase being analyzed.

6. Key difference 6: Community support and maintenance Both FindBugs and PMD have active developer communities, but FindBugs has received less active maintenance in recent years. PMD, on the other hand, has continued to receive updates, bug fixes, and new features from its community. This difference in community support and maintenance can impact the availability of new rules or bug fixes in each tool.

In summary, FindBugs and PMD differ in language compatibility, rule sets, analysis approach, IDE integration, performance, and community support. These differences make PMD a more versatile tool for code analysis across multiple programming languages, while FindBugs excels in providing efficient analysis for Java code specifically.