Edit:
Adding android:textstyle
to your xml should work.
In case if you are using custom font ensure that your font have italic
style. From the documentation of the setTypeface(Typeface) method:
Sets the typeface and style in which the text should be displayed. Note that not all Typeface families actually have bold and italic variants, so you may need to use setTypeface(Typeface, int) to get the appearance that you actually want.
If your custom do support italic, try this,
mTextView.setTypeface(Typeface.defaultFromStyle(Typeface.ITALIC), Typeface.ITALIC)