Loudness, K-weighted, relative to Full Scale (or LKFS) is a loudness standard designed to enable normalization of audio levels for delivery of broadcast TV and other video. Loudness units relative to Full Scale (or LUFS) is a synonym for LKFS that was introduced in EBU R128. Loudness Units (or LU) is an additional unit used in EBU R128. It describes Lk without direct absolute reference and therefore describes loudness level differences.
LKFS is standardized in ITU-R BS.1770. In March 2011, the ITU introduced a loudness gate in the second revision of the recommendation, ITU-R BS.1770-2. In August 2012, the ITU released the third revision of this recommendation ITU-R BS.1770-3.
The EBU has suggested that the ITU should change the unit to LUFS, as LKFS does not comply with scientific naming conventions and is not in line with the standard set out in ISO 80000-8. Furthermore, they suggest the symbol for 'Loudness level, k-weighted' should be Lk, which would make Lk and LUFS equivalent when LUFS indicates the value of Lk with reference to digital full scale.
Since ITU-R BS.1770-2, LKFS and LUFS are identical.