When the jivatman unites with Paratman, Ananda of spirit is experienced is this Moksha or Nirvikalpa Samadi what is the difference ?
There isn't any difference between the two. One who has enjoyed Nirvikalpa Samadhi has attained the Brahmananda (eternal bliss-the state of Moksha). Now, one who can repeatedly reach the same state again and again is called liberated.
Chapter 4 of Hatha Yoga Pradipika is specifically dedicated on the topic of Samadhi. The 2nd verse itself says:
Now I will describe a regular method of attaining to Samâdhi, which destroys death, is the means for obtaining happiness, and gives the Brahmânanda. 2
Verses 3,4 are as follows:
Rajayogah samadhischa unmani cha manonmani
Amaratvam layastattvam sunyasunyam param padam
Amanaskam tathadvaitam niralambam nirañjanam
Jivanmuktischa sahaja turya chetyekavachakah
Raja Yogî, Samâdhi, Unmani, mauonmanî, Amarativa, Laya, Tatwa, Sûnya, Asûnya, Parama Pada, Amanaska, Adwaitama, Nirãlamba, Nirañjana, Jîwana mukti, Sahajâ, Turyâ, are all synonymous.
So, in Yoga, words like Samadhi, Jivana Mukti (a state of being liberated while living) etc are all synonyms. And those words refer to Moksha.
For example, another word in the list "Laya" meaning "dissolution" is also used to refer to the same state of Moksha.