1. A display device comprising: a display panel on which data lines and gate lines are positioned; a data driving circuit configured to drive the data lines; a scan driving circuit configured to drive the gate lines; a timing controller configured to control the data driving circuit and the scan driving circuit; and a data modulation circuit including a local modulation circuit and a global modulation circuit, the local modulation circuit being configured to map luminance components of input digital video data to a luminance transfer curve for each pixel based on an average picture level (APL) of each pixel and perform a first modulation on the luminance components of the input digital video data so as to expand a gray level distribution of a specific portion of an input image, the global modulation circuit being configured to perform a second modulation on first modulated luminance components of the input digital video data so as to improve an entire contrast characteristic of an input image and supply second modulated luminance components of the input digital video data to the timing controller, wherein the local modulation circuit uses a luminance transfer curve that makes a dark portion of the specific portion to be brighter than before and a luminance transfer curve that makes a bright portion of the specific portion to be darker than before, and wherein the local modulation circuit includes: a representative value calculating unit configured to match the luminance components of the input digital video data to a plurality of virtual blocks obtained by dividing a display screen of the display panel in a matrix form and calculate an APL of each block as a representative value; a representative value interpolating unit configured to interpolate the APL of each block and convert the APL of each block into the APL of each pixel; a luminance transfer curve generating unit configured to generate a plurality of luminance transfer curves previously determined based on the APLs; and a data modulating unit configured to select a luminance transfer curve for each pixel based on the APL of each pixel or interpolate some of the luminance transfer curves to generate the luminance transfer curve corresponding to the APL of each pixel, map the luminance components of the input digital video data to the luminance transfer curve selected or generated for each pixel, and perform the first modulation on the luminance components of the input digital video data.