A few comments (and corrections of some misinformation above):
"... whether it is possible to view my pc's screen in a HD TV screen? " ==> Yes. Exactlly how you can do this depends on what inputs you have on your HDTV. Contrary to the comment above ["... All HDTVs come with a DVI connector ..."], very FEW HDTVs have such a connector ... and many do not have VGA inputs either. It's far more common to have either component inputs or HDMI inputs. In addition, the resolution suggested above is WRONG for all HDTVs. Most newer HDTVs require 1920 x 1080 (for a 1080p set) ... if you have an older 720p set you need 1280 x 720. In addition, depending on your specific HDTV, you likely need to use the video drivers' overscan compensation features to compensate for the TV's overscan (basically this means the effective resolution is slightly less than those numbers).
...To answer your question better, you need to provide the following details: (a) What video card do you have on your PC and what outputs does it have?; and (b) what make/model is your HDTV and what inputs does it have?
"... Do I need a special pci card? " ==> Not to use your HDTV as a monitor for the PC (but possibly to watch HDTV on the PC -- see my following comments)
"... is it possible to view the HD TV screen through my pc's screen in hd quality(the quality to remain basically unchanged)? " ==> Not without additional hardware. In addition, HDTVs do not, in general, have any HD outputs, so you have to use a loop-through device for the PC input. This means you can only watch ONE source on the PC ... and if you're using HDMI you also may not be able to watch it at all, since any HDCP protected content will refuse to display. If your source is connected to the TV via component inputs this is not an issue.
... Note that installing an HDTV tuner card (as suggested above) does NOT allow you to watch what's on your TV on the PC ==> it only allows the PC to act as a tuner for either ATSC signals (from an antenna) or ClearQAM signals (IF the tuner card supports this) -- unencrypted signals on a cable system (NOT any of the other HD channels you get from the cable provider). It doesn't do anything for Satellite programs (if you're using satellite) ... and does not allow any HD video inputs to act as a loop-through device for any other sources you may have for your HDTV.
With more specifics on your setup, I can provide better details on exactly what you need to do; but basically you can, in most cases, connect the PC's output to the HDTV with minimal additional hardware (perhaps a cable and/or a modestly priced convertor); and can connect a single HDTV input to the PC via a loop-through device [such as one of these:
http://www.hauppauge.com/site/products/data_hdpvr.html (best option in most cases) or these:
http://www.blackmagic-design.com/products/intensity/techspecs/ (supports HDMI -- but that also means you need to be concerned about HDCP issues)].
