LVTTL is a JEDEC standard that references the input signal to ground. The output switching range is 0.4 to 2.4V for 3.3V LVTTL and 0.4 to 2.2V for 2.5V LVTTL .
HSTL is a technology-independent interface standard for digital ICs that calibrates the input signal to a reference voltage rather than to ground. This calibration enables a smaller I/O swing and improves performance. HSTL was developed for voltage-scalable and technology-independent I/O structures. The nominal logic-switching range is 0.65 to 0.85V for 1.5V HSTL, resulting in faster outputs with reduced power-dissipation and signal-integrity issues.
For more details refer to the article below:
QDRII/II+ SRAMs adhere to the HSTL I/O standard.
Sync/NOBL SRAMs adhere to the LVTTL I/O standard.