The answer to this question is a little bit tricky, because it depends on how your computer is configured. Generally, the registry hive that is loaded first during Windows startup is the HKLM hive. However, if you have specific registry files that are configured to load during startup, that could override the HKLM hive and cause another hive to be loaded first.
Conclusion
The registry hive that is loaded first during Windows startup is the HKEY_LOCAL_MACHINE hive.
