The ordinary robes of priests are called Bigdei Kehunah (those with Ephod, Gold etc.) but the sacred ones of linen only, (Lev. 16:4, Ez. 44:18-19) to wear in the Kodesh Kodashim, what are they called?
They are referred to as בגדי לבן bigdei lavan, the “white garments.”
See Yoma 34b in reference to the preparations for the service in the Kodesh Hakodashim:
הֵבִיאוּ לוֹ בִּגְדֵי לָבָן לָבַשׁ וְקִדֵּשׁ יָדָיו וְרַגְלָיו בַּשַּׁחַר הָיָה לוֹבֵשׁ פִּלּוּסִין שֶׁל שְׁנֵים עָשָׂר מָנֶה בֵּין הָעַרְבַּיִם הִנְדְּוִיִין שֶׁל שְׁמוֹנֶה מֵאוֹת זוּז דִּבְרֵי רַבִּי מֵאִיר וַחֲכָמִים אוֹמְרִים בַּשַּׁחַר הָיָה לוֹבֵשׁ שֶׁל שְׁמוֹנָה עָשָׂר מָנֶה וּבֵין הָעַרְבַּיִם שֶׁל שְׁנֵים עָשָׂר מָנֶה הַכֹּל שְׁלֹשִׁים מָנֶה
And they immediately brought him the white garments, in which he dressed, and he sanctified his hands and his feet. In the morning he would wear linen garments from the Egyptian city of Pelusium worth twelve maneh, 1,200 dinars or zuz. These garments were very expensive due to their high quality. And in the afternoon he wore linen garments from India, which were slightly less expensive, worth eight hundred zuz. This is the statement of Rabbi Meir. And the Rabbis say: In the morning he would wear garments worth eighteen maneh, and in the afternoon he would wear garments worth twelve maneh. In total, the clothes were worth thirty maneh.