what is the difference between lcd and led tv technology????????

Now a days we are replacing LCD with LED

due to Led also available in nearly same cost

There are two technologies that are commonly called LED television. The difference isn't immediately obvious. 
The most common version of LED television during 2011 is actually an LCD television with an LED back light.

Other LCD televisions use fluorescent lighting behind the liquid crystal screen. In both cases, the light is allowed through the LCD shutters as required to generate specific colors and intensities in each part of the image.

The other version of LED television has no back lighting but uses sets of red, green and blue LEDs to directly generate the image.

One set of three LEDs is used for every pixel so an HD television will have (typically) 1920 x 1080 sets across the screen.

Because the display is generated with a single layer of components, the overall depth of the case can be reduced.

LCD shutters do not completely block the light generated from the back light and therefore have limits on how dark a pixel can become.

LEDs on the other hand emit no light whatsoever when they are off, so they have the potential for higher contrast ratios than LCD.

LCD shutters take a relatively long time to turn on and off.

Sometime, this can result in smeared images with fast moving changes to the image.

LED displays typically have a far faster response time and so should not suffer from image lagging or smearing.

The technology isn't new: Large television displays in arenas and sports grounds have used direct LED image generation for many years.

As LED devices are becoming smaller, so it is possible to build domestic size direct LED displays.

This technology has yet to mature but as it develops, it is likely to offer higher contrast displays and with the somewhat simpler components, perhaps lower cost displays as well. 

In terms of image resolution, both LCD (with LED and fluorescent lighting) and LED televisions will support similar resolutions so one will not offer higher resolution than the other.