Source: H I Sutton
A video capture posted on Chinese media on 9th September 2018 shows the Chinese Navy’s stretched Pr. 877EKM KILO Class submarine. The vessel has a hull insert approximately 15 meters long.
The increased space most likely accommodates an Air Independent Power (AIP) system to increase the submarine’s stealth in littoral environments. Other possibilities include a special mission compartment or cruise missile vertical launch tubes.
Read more at: http://www.hisutton.com/China_Stretched_KILO.html