We are normally using microsoft word, Wordpad or notepad for typing and setting Tamil using unicode font. In Scribus when copied the text from notepad, It shows the text correctly in Story Editor, but in the pages it is not display the text as per in the Story Editor.
This is encouraging.
Microsoft Word and OpenOffice.org Writer have a "helpful" feature: If the font you are using does not have a glyph they will substitute a glyph from another font that you have installed on your computer. Unfortunately, they do not tell you that they are doing this. They also do not tell you which font they substituted the glyphs from.
Scribus, on the other hand, will not do this in canvas view or in printing or export to PDF. The reason Scribus does not do this is because it is designed for printing on a printing press by a print company. It would be a disaster if the print company does not have the same fonts installed because the final print job would have missing glyphs. However, Scribus does substitute in the Story Editor. Therefore, the experience you are describing tells me that the font you are using does not have all the glyphs and Scribus is substituting the glyphs from a different font for the Story Editor, but is refusing to do so in canvas view or in printing or export to PDF.
I am guessing that if you use different fonts that have a complete set of glyphs for Tamil the problem will resolve itself. Unfortunately, I do not have a way to test this for you. I suggest you search for other fonts and try them out. I know there are some free open source fonts designed for Tamil that you should be able to find on the internet.