Love is not love till you give it away. Here 'till' is used as -

• Love is not love till you give it away.
- এই বাক্যে 'till' conjunction হিসেবে ব্যবহৃত হয়েছে।
- till এখানে ২টি Clause – ‘Love is not love’ এবং ‘you give it away’ কে সংযুক্ত করেছে।
- তাই এটা এখানে Conjunction.

• It's not a preposition because it's linking entire clauses rather than introducing a noun or noun phrase.

• Till (Conjunction, preposition)
English meaning: Up to the point in time or the event mentioned; until.
Bangla Meaning: যে পর্যন্ত না; যতক্ষণে না; না… পর্যন্ত; পর্যন্ত; অবধি।

Example:
- I went to bed at 8 last night and slept till 6.30. (preposition)
- He did not enter the town till it was nearly dark. (conjunction)

Source:
1. Oxford Learner's Dictionary.
2. Accessible Dictionary by Bangla Academy.