The extremely low latency of non-volatile memory (NVM) raises issues of latency in file systems. In particular, user-kernel context switches caused by system calls and hardware interrupts become a non-negligible performance penalty. A solution to this problem is using direct-access file systems, but existing work focuses on optimizing their non-POSIX user interfaces. In this work, we propose EvFS, our new user-level POSIX file system that directly manages NVM in user applications. EvFS minimizes the latency by building a user-level storage stack and introducing asynchronous processing of complex file I/O with page cache and direct I/O. We report that the event-driven architecture of EvFS leads to a 700-ns latency for 64-byte non-blocking file writes and reduces the latency for 4-Kbyte blocking file I/O by 20 µs compared to a kernel file system with journaling disabled.