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Wuxia is martial heroes, typically they max out at destroying mountains. Most of these stories take place in Jianghu/Murim (depending if the novel is chinese or korean), have Sect like Mount Hua, Tang Sect, etc. There's also stuff like qigong, and so on so forth. Xianxia is immortal heroes, now basically most cultivation novels, are xianxia as really and truly cultivation is more so a xianxia concept although it is interwoven into wuxia. The thing about xianxia which is really easy to distinguish is basically if there's extended lifespans, or well immortals its typically xianxia. When they're destroying planets and stuff etc its pretty clear its xianxia. Now technically xianxia only applies to cultivation novels which somewhat follow the Dao, and the basic realms such as Qi Gathering, Foundation Establishment, Gold Core, Nascent Soul, etc. Ones with unique realms are considered xuanhuan which afaik is basically just fantasy. But for all intents and purpose if they use qi, cultivate, there's immortals etc, just consider it xianxia. People confuse the two all the time and you will find people calling immortal cultivators wuxia but they're wrong, hope this helped.