There is a difference between Subversion Repository Version / Schema (1), and Subversion Repository FSFS Version (2).
FSFS, in contrast to Berkeley DB, or the developmental FSX, is a backend data storage method. FSFS is "a versioned filesystem implementation that uses the native OS filesystem directly".
(1): The Subversion repository version / schema is found in the [REPO]/format file. The current versions from the Subversion repos_upgrade_HOWTO document are:
SUBVERSION VERSION NUMBER SCHEMA VERSION
------------------------- --------------
Up to and including 0.27 1
0.28 - 0.33.1 2
0.34 - 1.3 3
(no released version used this) 4
1.4 - 5
(2): The repository FSFS format however, is found in the [REPO]/db/format file. The current versions are in the libsvn_fs_fs FSFS Structure document, in the Filesystem formats section. They currently are:
Format 1, understood by Subversion 1.1+
Format 2, understood by Subversion 1.4+
Format 3, understood by Subversion 1.5+
Format 4, understood by Subversion 1.6+
Format 5, understood by Subversion 1.7-dev, never released
Format 6, understood by Subversion 1.8
Format 7, understood by Subversion 1.9
Format 8, understood by Subversion 1.10+
Also, as of Subversion 1.9 you can now use the svnadmin info command to gain both the Subversion Repository Version / Schema, as well as the Subversion Repository FSFS Version (See: Subversion 1.9 Release Notes - FSFS Improvements as well as Subversion 1.9 Release Notes - svnadmin Changes and Improvements)