I noticed the following as an example:
In[14]:= im = ExampleData[{"TestImage", "Sailboat"}];
ImageValue[im, {100, 88}]
PixelValue[im, {100, 88}]
Out[15]= {0.411765, 0.364706, 0.352941}
Out[16]= {0.509804, 0.537255, 0.545098}
Sometimes the difference is almost negligible, other times it is quite significant.
In the docs, both claim to: "gives the pixel value of image at position pos."
Can anyone explain why this different results?