Nirvana has different meanings, but the two primary meanings are: 1) extinction, your material existence is being extinct; 2) bliss.
The Supreme Lord clearly is the purpose of bhakti-yoga, because only if the Supreme Lord is the purpose of this yoga you will achieve the state of complete bliss and extinction of material existence. Other kinds of yoga are talking about paramatma or impersonal Brahman. Only in the pure devotional service the Supreme Lord is the purpose of yoga. If the goal is not clear, there is no chance that you will reach this goal. When people usually do some spiritual practice they don’t know what they want to achieve, therefore they don’t achieve the Supreme Lord and this experience of nirvana is not there. If you don’t experience the direct contact with Bhagavan while you are serving Him directly, then basically you are fallen, because you will come back. Because we are people we need to have personal relationships.
There are different engagements of bhakti-yoga. There is sravanam, and when you are a little tired of sravanam you can have kirtanam, when you are a little tired of kirtanam you can have some smaranam, and so on. Bhakti means to engage your senses in service to the master of the senses. Therefore you realize your complete potential in performing bhakti-yoga. That is the full existence in the spiritual life, all other kinds of so-called spiritual existence are not really complete and therefore they are not so attractive. Bhakti means krisna-bhakti, it engages all your senses in a very blissful way that you never want to go back to this material world.