All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad  is His slave and Messenger.
  is His slave and Messenger.
If by 'Moulavi' you mean a preacher or a caller to Islam, then there is a difference (from an Islamic view point) between a Mufti and a preacher. 
A preacher is required to convey [to people] the knowledge which he knows even if it is only one verse from the Quran or a single Hadeeth. The Prophet  said: “Convey on my behalf even if it is only one verse (from the Quran).” [Al-Bukhari]
  said: “Convey on my behalf even if it is only one verse (from the Quran).” [Al-Bukhari]  
As regards the Mufti, there are some conditions and requirements which should be met in order for one to be eligible to issue a Fatwa and become a Mufti. These requirements were mentioned by the scholars  ; in this regard please refer to Fatwa 89691.
; in this regard please refer to Fatwa 89691. 
Therefore, it becomes clear that the difference is based on rules from the Islamic legislation.  
Allaah Knows best.